No need to assume when Blizzard had all the information here.
One of the pinned topics:
One of the paragraphs in that topic:
Won Arena but shows as a loss?
This is usually due to a disconnect while searching for a game or when you cancel your matchmaking. The game places you into a new match instead of reconnecting you into the old one. More details can be found here: https://us.battle.net/support/en/article/17276?blzcmp=app