when LW blue is full, LW orange has its cap raised, when orange is full, blue has its raised, that way they can both grow to much higher levels.
But, as one arena empties, the cap in the other drops and that way you can have more players in an arena than the cap allows. No one gets booted, it just wont let anymore in.
Check again later cap still 200, 219 in there still can't get in
I figure a player beat you into the arena (making it 200 players), otherwise it would have let you in.