he probally one his last 20 matches. its not an average of wins and loses, its how well you have currently done. i have seen a warlord with more loses that wins. because he had been doing well currently.
lets say lvl 23 life dude steps into arena. sucks at it, gets like 60 loses.
then he comes back at like fifty, totally starts destroying in the arena. gets like 65 wins and a few loses. and now he is a captain. because how well you are currently doing.