Money.

Your manager is trying to give you as little money as possible.

Every year, upper management gets on a kick where labor is very important to them: how much is spent, what percentage of the sales it makes up, how they can spend less.

They will keep the studio understaffed and try to send you home the moment you aren't needed. If you're on flex, they really don't want to bring you in. If you're on coverage, they really don't want to have to give you the flex help and will want you to shoot sessions 30 minutes apart by yourself.

Part timers, expect to get 15 hours max no matter what your were promised.
Full timers, they are just dying for the opportunity to cut your hours if your sales numbers fall. It doesn't matter that being under 30 hours a week puts your health benefits in jeopardy. If you want health benefits, you better get your sales numbers up.