So what if the buyer did know there was a difference in price?  What does that have to do with it?

Buying a car is basically a game - the buyer tries to get the vehicle for as little as he can, and the seller tries to sell for as much as he can. 

If the buyer was trading vehicles for the same price, he got a deal.  Even if he knew that the blue one had more options, and should cost more.  If the dealership made a mistake on the contract,  that is the dealer's fault, not the buyer's fault.