Depends on the person but, as the top comment says, many of them are actually that dumb. There's that joke, think about how dumb the average person is, and then realize half are even more stupid.

Yup, some people actually couldn't understand "per capita" if their life depended on it. Which is especially funny, considering they can seemingly intuit it if it argues in their favor. They still don't understand it, but they'll accept the argument if it proves their point. Even if it's the exact same math that they can't understand and think you'll lying about, in the other context.