Myself and three friends started playing arena last night, specifically the mixed game-mode playlist (I forget the name). We had each played a bit of warzone previously, one guy had played a few FFA arena matches. We played all 10 of our qualifying matches together on the same team. One guy disconnected in one of the games, but we still ended up losing that game. After the qualifying matches were completed, two of us were placed in Platinum 1, the other two were placed in Gold 1 (including the guy who disconnected one game).
I would have expected us to all end up in the same ranking bucket if wins and losses were the only factor, so there is either a bug or something else is being considered.