Different structure I believe. I am close enough to the car industry to know that their month end bonuses are most of their salary. So if they just need one more sale to hit their bonuses, they offer part of their bonus off of the car as incentive to get you to buy. After all, a slight bonus is better than no bonus even if the customer benefits the most out of it.
So month end is basically what the General Manager is willing to do to make the store targets and what the sales person is willing to do to hit their month end bonus. Usually the combination is quite decent.
Not sure how watch sales are structured but all of my guys tell me they don't get bonuses. I call BS but I don't have any connections there to know for sure. Since they seem to act the same early or late month, I don't think it's quite the same.