Plus, factor in that dealers can trade cars/allocations too. Dealer X only has 1 235 and two buyers. Rather than send buyer #2 to Dealer Y, X may swap (of sorts) a 435 for the 235 and keep buyer 2 at his dealership (plus likely for the next time #2 buys a car).
But if they are only allocated 3 235's a month, and all the other local dealers are only allocated 2 235's a month, there are not that many out there for people to choose from. If the demand is high (often when a new model comes out), then each dealer is going to hold things tight and want the most our of each car. If you're a cheap bastard and trying to lowball, they can wait another 2-3 weeks and sell the car to the guy willing to pay top dollar. They can only sell 3 235's a month, so they want top dollar for each (knowing they'll likely get it this early in the model).
If the model has been around a bit, it's more likely a guy can do an ED, and that car will not come out of the dealers allocation. So, if he's allocated 4 Z4's a month, I can go in and order my ED Z4, offer $500 over ED invoice, and he's apt to take it. My ED car doesn't not subtract from his 4 allocations. So, he can still get top dollar for those by the guy who isn't willing to wait 4+ months for a car.
'15 3500 HD Dually