This could be related to the situation where people win in Arena, yet their run ends, due to a disconnect during matchmaking:
So if that happens when you play ranked, you could be essentially placed in two games at once, one you loose because of the disconnect, one which maybe you win. Once your game is won, the client checks how many stars you now have, and updates the display. In this case, you may be gaining one star from the win at the same time as loosing one for the phantom disconnected game.
Did you have any connection issues when that happened? Or, possibly, did you enter the Play queue, then abort before it found you an opponent? There may be a very small time window where the game finds you an opponent, but the client does not yet disable the Cancel button, so it could lead to the same situation.