why is annette's name Dee Dee in this but Delores in Beach Party?

I think "Dee Dee" is generally a nickname. So maybe the character goes by that nickname sometimes, but not all the time.

Anyway, I don't think these "party" movies were all supposed to be in some kind of tightly-woven continuity in the exact same universe. They were just loosely connected by the theme of partying at the beach.