An example:

In level 1 there are: 10-14 (3stars), 15-19 (4stars), 20-24 (5stars) and so on...
In level 2 there are: 10-14 (2stars), 15-19 (3stars), 20-24 (4stars), 25-29 (5 stars) and so on...

So if you have a player with quality 21 and you are in level 2 it looks like he have 4stars. For your friend in level 1 it looks like the same player with quality 21 has 5stars! It depends on what level you are in.