This means the character you left in the arena lost matches versus other players. For example, when you played yesterday and won matches to gain MMR 927, you likely defeated other players AI taking MMR points from them. In essence, you are fighting the AI character and settings they (opponent) last used when they exited the arena. This is the same for your character - the settings, gears etc you last used when you exited the arena will be what is used for your character to fight players when they log in to play the arena.
Hope that helped.