Mere stupidity doesn't explain it, as it would cause people to assume themselves mistaken as often as they assume the employee is. As clearly demonstrated, there's a gigantic bias blind spot regarding who's at fault (one capable of instantly switching around memories), which I expect isn't there (as much) when these people are dealing with peers or superiors.

Or to take another example, I can't find the reference: a famous (former?) con man said that one key to keeping the mark from thinking clearly is getting them to think they're getting the advantage of someone dumber or otherwise lower status.

Obviously, all anecdotal, but strong enough, and not paired with corresponding evidence in the other direction.