I have a relative who once thought the entire continent of Africa was one country.
It's funny how different dumbness bothers us differently. I am fine with doing math but when people say they can't do it it doesn't bother me. But geographical dumbness irritates me a great deal. Just the other day someone said to me, "aren't India and Indonesia the same country?" and they were at least 40.
It's true though that people absorb basic knowledge differently. One person can watch a bunch of WWII movies in childhood and know roughly what years WWII took place in. Another person can have no idea. I think I was always facts oriented and absorbed all kinds of trivia and details from vegetating in front of the tv as a kid.