People seem smart when they know something you don't know and want to know. They appear dumb when they don't know something you consider easy.

If you use those definitions instead of some hypothetical absolute standard of intelligence, things seem to be a lot clearer, and you no longer have trouble understanding how "someone so smart could be that dumb" or things like that.

Every person who is X years old has had the same amount of time. What they've been able to do with it is another matter entirely.