First, all the opponents in the arena are bots, you don't play against real opponents or real decks. So seeing lvl 13+ opponents doesn't mean anything. Second, Spheedy published the percentages at some point on the discord server, but it was something like 2% chance to get a red chest in lvl 9 arena (unless you are out of slots, in which case it is much higher). Third, the prizes indicate you can get multiple types of souls, ie a red chest will give you arley or triplity souls but can give other lower level souls as well- in theory you could get 6 different kinds, but it's usually less. Since the lower level souls are useless, none of this matters. No other prizes are awarded for arena chests, other than viagra for the three star bonus chest and of course, filling requirements in events. Fourth, the percentage chances are actually identical in league 9 and 10 (again, based on numbers posted by spheedy a while back in discord), so the answer to that doesn't really matter. I'd guess the league you are currently in when you open the chest, as that's how the three star bonus chest works.