Here's what I think happened:
1. when your 37.5K award ticket was cancelled by mistake, your miles (37.5K) were refunded, as well as taxes. It appears your booked MileSAAver one way and AAnytime the other way. There's no charge for changing an AAnytime award, I'm guessing that is why you weren't charged the $150.

2. When the PLT agent rebooked your gf's reward ticket, it appears he simply modified your previous 25K award, the one that you had intended to cancel, but was not cancelled by mistake. He may have rebooked your gf in Y because it was the easiest and fastest thing to do, given the time constraint-trying to get your gf a seat on the plane that is departing. Or he may have made a mistake, instead of booking an RT MileSAAver ticket (50K miles), instead he booked her in Y.