And when you agreed upon the bet, was there any explicit mention of using the "officially tracked NHL goal differential"? You've implied that term into the agreement, but at the time, nobody specified.
This is where it becomes clear to me. The term "goal differential" is a term invented by the NHL that refers to the actual stat they track. If you are using any modified version of the stat then you'd have to clarify that... otherwise you'd have to assume you're referring to the actual stat.
Just like +/-... they don't count power play goals in +/-. If you make a +/- bet you'd have to specify if you want to include power play goals because the stat itself doesn't actually include power play goals.