Sometimes they are "wired" differently. They can process and absorb information in a more efficient manner. This is a natural tendency, they are born like that. Others, through training have learned to think forensically. To filter out pertinent and relative info and discard the filler. Lawyers, doctors, detectives, scientist.
Well, very simply, because some people ARE better than others. You would be correct to think you are a better person than the nearest homicidal maniac rotting in a cell. The problem isn't when someone rightfully understands that they are better than someone. The problem is when someone judges themselves to be better than someone, when they really are only about the same or worse than the person they believe themselves above.