I've never clicked/used the MAX becasue that would scare the shit out of me, so I'm guessing here, it depends on how the process has been programmed.

If (a) it sends to the server "I want to use the MAX" and then the amount is fetched server side or (b) it sends the exact amount itself as previewed in the input box.

(a) If it sends "I want to use the MAX" you might have been credited the Manchester winning right between the timeframe you clicked the MAX button and the instant the data reached the server and it was processed, so at that time it processed the MAX including the Manchester winning.
If it works this way you may have been very unlucky.

(b) If it sends the exact amount as it has been put by clicking the MAX button there couldn't be any misinterpretation on what happens server side. If it works this other way, the 0.04xx BTC amount was put in the box and you didn't notice because maybe the Manchester winning was just been credited to your account.
Here you must be honest with you: did you double check the sum you were betting before clicking the submit button - that might have been 0.04x BTC because it was just the winning being credited - or rather having done that same action hundreds of time you skipped the check part and went straight to the bet button?