All motors vary a little from company to company but here it is from Novak...
21.5=1800kv
17.5=2200kv
13.5=3300kv
10.5=4200kv
8.5=5000kv
7.5=5800kv
6.5=6400kv
5.5=7400kv
4.5=9000kv
3.5=10500kv
The lower the turn or the higher the kv... the faster the motor
The higher the turn or the lower the kv... the more torque
Think of it as s.a.e. versus metric
As long as someone is not running a faster motor than the highest 13.5[or 17.5 or 21.5 whatever the class] no one should be complaining...