Why do people think I am smart when I am not?

Throughout my life many people have thought I was like a brainiac. I spoke early and people just said I sound smart. My grades were ok but my IQ is average and despite being a pretty decent writer; I am nothing special. As a kid, I had glasses and lots of people called me a nerd. I guess I am since I find science and Star Wars to be fascinating. Yet truly smart people have also called me an idiot when I was exposed to them; even some people on this forum. I don’t know what to do? Should I keep pretending to be smart like a lot of people think or should I keep telling others I am not the sharpest tool in the shed. I’ve been doing that with some of my professors (I am in grad school) and told my archivist professor that the work is challenging for me.