I eliminated the last of the opponent's fleets. The VICTORY! text came up, the achievement counter +1'd, and the music played. While the music was still playing, the opponent left the game (via MENU -> Quit). On my end, the music restarted and the achievement +1'd again.
So what's happening is the game detected the last remaining opponent leaving and awarded me the victory, while I had already won, but still technically 'in-game' as we had not yet ended the game by returning to the lobby.
Hmm, I can't seem to reproduce this. Does it happen reliably?
Hmm. I don't recall noticing before, I just happened to be paying attention. Here were the EXACT conditions, as I remember them:
Six human players. No observers. Four people died, only me and Ark left, but the other 4 players are watching. I captured the last planet, two turns later his last fleet strikes one of my planets. My fleet there wins the fight, the VICTORY screen appears. My achievement +1's. Ark leaves. It SOUNDS like the victory music restarts (could have been my mind playing tricks?) but the achievement definitely +1'd again.
I may or may not have been the host.