There are two common mistakes in self-judging that are interesting to analyse together.
The Dunning-Kruger effect is a cognitive bias that warps our ability to judge our own competence. To put it very bluntly, stupid people tend to think that they are quite brilliant, because they are too stupid to tell that they are, in fact, stupid. (Don’t worry, I know it’s a bit more subtle than that.)
The Impostor syndrome is a concept describing high-achieving individuals who are marked by an inability to internalize their accomplishments and a persistent fear of being exposed as [they were] a "fraud". [...] Despite external evidence of their competence, those exhibiting the syndrome remain convinced that they are frauds and do not deserve the success they have achieved.
Image from The Curious Case of Impostor Syndrome
Put two and two together
The Impostor Syndrome is a perception bias for which brilliant people think they are cheating others instead of recognise themselves as high performers. A very simple explanation could be that people do not tend to underline great achievements to others because it seems them obvious and moreover many people are affected by Dunning-Kruger effect which makes to value themselves better than they are in real.
Looking at the graph above, the most interesting thing is that almost all the people value themselves above the middle (>50) but in reality the score are distributed between 10 and 90 c.a. and only less than half will pass above the middle (>50) and more than 25% undervalue themselves. This means that about 60% of people overrate themselves.