I also noticed that in every exercise you get over the 50% rather quickly. perhaps there are a lot of dead accounts of people that logged in only few times counted, otherwise it is a bit weird. also, is there a reason why we can't see the stats of other people?

I'm on Level 429 of Chordelius and on a Percentile of 99.83%. I would hazard a guess that the Percentile figure is not to be taken absolutely literally and is perhaps skewed upwards in order to encourage participants?
Also, to make substantial progress in a game you have to play it on its own over and over – and I would guess some games are not played nearly as much as others. So, you could probably get a high percentage relatively quickly on the lesser-played games. Chordelius is so much easier if you play a chordal instrument as opposed to drums. But it's worth noting that when you get into the high 90%s you often have to clear many Levels to move your Percentile mark. I was stuck on 99.94% of Interval Barks for one thousand Levels – Level 334 to Level 1,334!