Percentile?


Also, I'm around 1800 in [another site -- VP] and I have the same percentile, this make the thing even stranger

https://support.chess.com/article/857-what-does-percentile-mean
Most members are casual players. Even in OTB chess, there are a lot of beginner player, or casual tournament players, so it doesn't take a high rating to be in the top 20%.
Ratings across pools are not normally comparable, since different sites have differing starting ratings, but the percentile numbers, in pools of sufficient size, likely have a better correlation.