Why is it different? Are the NIV and KJV indicating that God is not the "He" that began a good work?
A: This is a really great question and actually touches on a very important aspect of Bible study that is often neglected. I say that up front because you may be disappointed with how simple of an answer this really is!
At the outset, I must say how thrilled I am to hear that a part of your "studying" method is to compare your passage in different translations. This is a really excellent practice. It is very helpful. As you've noted, the translations often read slightly differently. In this particular case, you'll see that the difference isn't substantive. But in other cases this practice of comparing translations will really bear good fruit in your study. So keep up the good work!
Why is comparing translations such a helpful practice?
We must understand that the original documents were written in three different languages. The Old Testament was written primarily in Hebrew, with some portions in Aramaic. The New Testament documents were written in Koine Greek.
If you've ever studied languages, in even the briefest fashion, you know that languages do not have an exact "one-to-one" correspondence. That means, that there is not an exact word in English to match every word in Greek. Most words in different languages don't cover the same range of possible meanings.
Part of the beauty and genius of language is that it is flexible. Adaptable. Capable of expressing both the simple and the complex. In different ways.
Here's an example from my son's favorite joke:
"Oh, don't cry! It's just a joke!"
You see how that plays off of your understanding and fluency in the English language? To translate that into other languages may be much more complicated. The "punch" may simply be lost in translation.
Since the Bible contains such a diverse range of writing styles and genres, we really do see everything from riddles and poetry to historical narrative to fictional stories being told to illustrate poignant truths (i.e. "parables").
When comparing translations, you may find that the translator(s) tried their best to bring out some of the original nuance but their focus on one aspect necessitates their neglect of other "translational" possibilities. Comparing different versions will help you to identify these areas.
A quick example, compare John 3 in the following versions:
KJV Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God.
NAB Jesus answered and said to him, "Amen, amen, I say to you, no one can see the kingdom of God without being born from above."
NASB Jesus answered and said to him, "Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God."
NET Jesus replied, "I tell you the solemn truth, unless a person is born from above, he cannot see the kingdom of God."
You'll notice that these translations read very similarly. But there is some disagreement between whether or not Jesus said "born again" or "born from above." So which is it?
In this case, the original Greek contains a word that is ambiguous. It contains both as a possibility, even though the difference in the English words ("again" and "above") are not very similar in meaning at all! Comparing translations will help alert you to possible ambiguities in the text so that you can study them more closely.
A second category of differences are not in the words used but in the grammatical decisions. This is the category that the Questioner's question falls into.
Something that most Christians that I know have never done is read the Translation Note in the front of their Bible. This section explains the philosophy of the Translator(s) and also explains some of the translational conventions that they use throughout.
In the above question, the differences arise between the grammatical convention of capitalizing pronouns (He, Him) when they refer to God in the NASB and not capitalizing pronouns (He, Him) when they refer to God in the KJV and NIV. This decision is consistent throughout the translation. So, consistently the NASB will capitalize these pronouns. Every time they are referring to God. But the KJV and NIV never capitalize these pronouns. Even when they refer to God. Unless, of course, other grammatical rules would necessitate capitalization (e.g. the first word in a sentence).
This grammatical convention of capitalizing He/Him when referring to God is customary for some. But is not necessitated by grammatical rules. It is a stylistic question. Not a substantive one.
Therefore, to answer the above question directly: the lack of capitalization is not making any commentary on the NIV or KJV thinking this does not refer to God. The difference is merely in style.
There is another translational convention that is used in the above example which many people may not notice. I want to briefly point it out.
Did you notice that the first three words ("For I am...") were italicized? Often in writing, italics are used for emphasis. But that is not what the NASB translators are trying to communicate. Instead of emphasizing these words, they are making it explicit that these three words do not appear in the original Greek. They are being supplied for context. This helps explain why this part of the passage reads differently than in the KJV and NIV. The translators of the NASB want you to know they think these words are rightly supplied because they are implied in the original. But they also want you to be aware that they are adding something that is their opinion.
A final type of decision in this last category includes the decision of translators to end and begin new sentences in English in different places. This is a result of the Greek tendency to use many run-on sentences. These are displeasing to our modern English sensibilities. The writings of the Apostle Paul are a good example. There are some cases where a single sentence spans nearly an entire chapter! Translators feel compelled to break this up. Comparing translations will expose that these decisions are not uniform. To gain a better understanding of Paul's intent (or any other author in Scripture), it is always a good idea to study these passages by reading them in several different translations. Especially across different translational philosophies.
Translational philosophies range dramatically. You can have "literal" or "word for word," which is sometimes referred to as "formal equivalents" or "wooden" translations. You can have more "idea for idea" translations, sometimes referred to as "dynamic equivalents," or "loose" translations. There are also "paraphrases." These are not really "translations" at all but actually closer to an interpretive commentary. Paraphrases (e.g. The Message, NLT) should never be your primary Bible. They can have an appropriate place being read alongside genuine translations during your study process.
To recap: the difference you noted does not imply that the KJV and NIV are saying that it's not God who does the work. All of these translations indicate the same understanding. God is the one at work. He will complete the work He has begun.
As an action step, I encourage all who desire to understand their Bible as well as possible to take a few minutes to turn to the front of your Bible and read the Translational Note. See what the philosophy of the translator(s) really is. Learn what conventions they employ to communicate the translational decisions they have made.
May the Lord bless you as you continue to seek Him and follow Jesus in this life!