Hi Daniel!
In
this post Jack explains a few points about League filter and your question is included!
Be careful on reading that thread, because it can be outdated.
Jack
[...] which is why the leagues with 31/32 teams or 15/16 teams no longer show up. This is basically to prevent multiple people joining a league in succession and it being full by the time someone tries to join.
Basically what I try to do is keeping the Rookie tier with 14 teams.