The simple fact is that there are two methods of calculating goal differential. There has to be an agreement on which method is used, otherwise there was never really an agreement on a bet. Neither one is inherently the correct one to base a bet on.
ME is simply using the method that Sensible used at the time of the bet. That was the only method this bet could have been based on.
Now maybe Sensible did not realize the stat he was using, but he sure made it seem like he did to ME. That is one of the first rules of contract. Sensible intention doesn't matter. It is what a reasonable person would believe his intention was based on his actions. He flat out used goal differential including shoot outs. There is no other way to interpret that.
Me could be a nice guy and let him out of the bet if he wants to (doesn't sound like he'd get paid if he won anyway). But the terms of the bet are what they are.