This is not rocket science. Its like two shoplifters enter a store, one black and one white, and the person monitoring security is white and believes blacks are more likely to be criminals than whites. The fact that the person believes blacks are more criminal means that blacks get watched more and consequently black shoplifter gets busted and the white shoplifter gets away. Its called a self fulfilling prophecy. If not for the bias of the security person, they would have caught the white shoplifter. However, by focusing on blacks they catch more blacks thereby fueling there belief that blacks are more criminal.
People assuming whites are smarter means that they don't try to push these higher priced loans upon whites like they do upon blacks. However, if they tried to push them upon whites as much as blacks whites would fall victims at similar rates as blacks, but because they don't try means that black fall victim and you have situation like we have at the bank.