Google voice does not supply caller-ID name service. They only supply the number.
Why, is unknown. It would seem simple to supply both if you're supplying one, but Google is very bad about answering questions.
One thing that you might try is to enter the names and numbers of your frequent callers into your phone's memory/director/what-have-you. For some phones, the number will be matched and the directory identity will be shown/spoken. This works with some AT&T phones, at least, and may be what you are actually seeing.