It may be because people think about their own area of expertise. So they think, "Am I smarter at stopping criminals, working out which medicine to give, making a wireless module, re-doing the plumbing in a building, calculating the value of a derivative, than other people?" The answer is yes. It doesn't mean they're smarter overall than other people.

When I finished undergrad and started working designing power controls from the silicon device fabrication industry, my first boss told me the smarter he gets the more he realizes he doesn't know. I thought of it as a platitude, but I believe it now. The more you drill down, the more interesting questions you find.