Many rebates, such as the college grad $500 rebate, are not elligable with X-plan pricing.
Dealers make $150 on each x-plan car they sell. I proposed to my salesman that he sell me the car at a "market price" of $150 over the X-plan price; I got the $500 rebate (minus the $150, making it $350) and he still made his margin. Easy as pie.