It's called tip share; there's some inequities between the "back of the house" and "front of the house" when it comes to every restaurant. No matter how busy you are, the back of the house (along with some front of the house like bus boys and hosts) gets paid the same amount of money regardless of how hard they work. The wait staff gets paid more the busier they are since they are tipped employees, and more customers means more tips. To incentivize all employees to be more efficient, the restaurant takes some of the tips and redistributes it to all employees. Often they just take a percentage of credit card tips.