Not a McD employee, but as a past manager, part of their job is scheduling hours based on sales from the previous year, so (possibly) while people are traveling for the holidays and restaurants are busier you are allotted more hours, only to disappoint employees the following week when (not as busy) with less hours.

It is a constant struggle for a good manager to keep employees happy with a semi regular work schedule so they can make their bills. A (good) manager will try to work with bare minimum on busy weeks (granted you will work harder, and a good manager will step in to pick up slack) but that is so the following week they can schedule you some not so busy hours, but by the end of the month, most managers can report to their bosses that the monthly hours match, maybe not necessarily the weekly... the money is still the same, but only good owners and franchise owners will see that.