re: WTF did the Professor and Mary Anne do to not get equal billing?

Watch the pilot. Mary Ann was not in the episode. There was a professor, but he was not played by Russell Johnson. There was a Ginger, but not Tina Louise. If you read about the history and Tina Louise's complaints, she probably got in the credits even though she wasn't in the pilot because she was well-known (she claimed that she was conned into joining the show because she was told she would be the star) and Russell Johnson and Dawn Wells were unknown.