If its not a bug then its not going to be worth playing the arena.
I did a battle where I had my opponent beat in every category and still lost. That loss cost me 31 mojo. He had 2391 hardcore and I had 2548. He had 1956 charm and I had 2249. He had 1369 know-how and I had 2133. His ego was 19905 and min was 23983. We are both level 75.
How the heck did I lose?