They will give you the minimum amount of hours in order to minimize their employee costs(and maximize their profits). They way they make money is through having an "account" with a venue. Sometimes you will be asked to start work at 10pm and only get a few hours (sometimes you'll make less than $40/night after taxes), which isn't always worth it when you've put on a suit and traveled to work somewhere. Also, they will lower your hourly rate without notifying you.