I know I only posted ten minutes ago, but I think I've worked out what happened.
I had a B3 game start at 6:05 GMT, I made the bid after the game had started, but I had not yet received the money for winning the game (I was not aware at the time I was going to win), but I think even though the funds were not on my economy page, when I bid they must have been officially included, so I was able to bid more money than I actually had at the time.
Therefore anyone involved in a cup or B3 game can find out if they win or not by bidding more money they have on a player. (Do so at your own risk, I'm not really recommending such a thing)