not just holdback, some manufacturers offer hidden stair-step incentives that once hit, are retroactive to all vehicles sold that month. That's why you can usually get the best deal towards the end of the month, when the dealer is willing to sell below cost to get that hidden incentive, sometimes worth over $100,000 depending on volume.
This American Life made a whole episode in a dealership which had that exact problem: they had to sell 129 in a month to get a bonus from the manufacturer.
"The last day of the month continues and the truism is accurate: some people get great deals because it’s the end of the month and they have to hit their goal. When you look at the numbers, the average car they sell in the last two days actually loses money."