Could be a few players were saving gems for pulls and the new girls pulled them in

Could be learning to play

Could be hoarding

I've actually jumped a few spots where I evolved 3 girls in a matter of days and then powered them up to the 30s with saved Manyu

In my early game i just leveled every Girl to 40, since after that begins the jump in needed exp to further level up. After that there were a lot of evo's for my 4*'s, which made me jump in Power really fast. Still don't have Level 70 Girl for the H Squads, but that is never needed, if you look at the composition the H Squad you choose consits of.