Many times I have made the mistake of calling someone stupid who really was not dumb, they were just uninformed. I think that most people either just have the wrong dataset or the wrong worldview.
For instance, most Americans think that suicide bombers are idiots. I would disagree… if you were in their position you may make the same decisions. Imagine being trapped under a totalitarian government and your only hope is religion and you have not been educated well enough to understand religions scientific fallacies…. Of course some suicide bombers are actually stupid or retarded, but I would guess the majority of them are just desperate and ill-informed.
A lesser example might be your boss. You spend all day digging into the same information and have a thorough understanding of it, whereas he can only devote 1/10th of his attention to the same data. Of course you understand it better, but that doesn’t make him stupid, just uninformed.