It is explained in one episode by David (Kevin) himself. He either says that it is a name they just started calling him and it stuck or it was his middle name. I can't remember which it was, but it was taked about real briefly in an episode. I think truely, the producers wanted his name changed because "Darlene & David sounds better than "Darlene & Kevin"

That's not what I thought! I seem to remember Roseanne saying to Dan once that "David isn't even his real name, Dan! Darlene just liked the name David better!" (not exact quote, but damn close, lol)

I don't think that she was even talking to Dan when she said that. I think that it was Jackie or someone, wasn't it? Anyway, yeah, your quote was pretty close but it was really something more like:

"David's not even his real name! It's just something that Darlene made up for him because she liked it better."

I think that she said that when she was talking about how much control over David that Darlene had in their relationship, or something like that. But of course, I could be wrong, so someone can correct me if I am.

I believe Roseanne was being sarcastic. imagine that!LOL
I think it was just and error made in writing, several shows have inconsistantcies.sp? Heck, Ethel from I love Lucy had three different middle names mentioned at different times. B/c David's own mother called him David the few times she was on there. I don't think she would call him by a name Darlene made up.