How do you allocate a $x promo for selling Y cars in a month, when some of those cars were unpopular crap and others sold themselves?
A dealer for example will get more back per car sold, if they have the most modern building style. (a way manufacturers entice dealerships to remodel to latest corporate recommended aesthetics/experience).
They also pay more if your dealership is stand alone. A joint VW/Audi/Porsche dealership will end up paying more for cars (getting less money back technically) than a standalone dealership.
I could go on and on about the various metrics for getting more or less rebate money back.
Would you really pay more for your car because a dealership paid more because they hadn't remodeled recently? Of course not. You compare to what others are doing and that should be enough, unless you really want to try and randomly guess just how low they might go that day and walk and try again everywhere?