They do that so that they can offer more in px if you have a car that they would like for stock. WBAC aren't all that far away when it comes to the true value of retailable cars. However, falsely inflated retail prices at the dealers, mean that some customers assume that equates the cars are holding their values well.
When I'm looking to buy a used car I get both valued at WBAC to give me the true difference between two. Then I know what I'd consider to be an acceptable price to change.