Thinking about this some more, it's likely a stuck transaction on the invader side. The invader won, the win message only got transmitted partially, causing it to get credited as a single point win on defender side due to corrupted data. Processing aborts afterwards, without clearing the transaction. The transaction gets picked up again every time the invader completes another fight, with the same processing result, so a very active player will seem to fail a lot. Should be visible in the server logs.
Just gut feeling though, may be miles off the target.