The higher division you are, the harder to get an XP

Answer 1

U mean we lose more xp when lose a challenge

Answer 2

It's absolutely normal and the same applies for most games too. Have a read on the Elo Rating System if you're interested as SoloLearn implement it according to the FAQ. 😉

Answer 3

https://en.m.wikipedia.org/wiki/Elo_rating_system