Simple, when you enter arena at just the right time, you triggered a bug that makes the game think you haven't completed all the battles.
Or in other words, bug.
Not really an expert when it comes to bugs, but maybe Raul will arrive here.
When I enter Arena i saw this:
Yeah, just a completely normal picture BUT i already does more than 5 win and already got 20 arena coin and some gold. Somehow game decided to give me 4 more turns to get extra coins?
(I'm not complain about it, i just want to know why this happen).