I don't think it's that hard. For people who solo queue maybe, they only get a picture of their skill bracket. Play with friends and it becomes more clear. I have friends who consistently go 0-10 in games they play with me, then they go solo queue and go 15-0 and laugh at just how bad everyone in their bracket is.
If you take the page doctrine into account, even as high as page 50-100, which is still almost the top 10% of the players, people are pretty horrible. Then you take a second to realize there are 700 pages of people at an even LOWER skill bracket than them.
I mean, if there are 800 pages of games, and we assume average means 50%, if you're above page 400 you're above average.