Well, with that story setup, you can either do A topping B or B topping A. Not doing either probably kinda would ruin the punch line.
So it's a 50% decision and there's arguments for either. If they had done the other way around people would argue about the same thing in reverse, guaranteed - which is vaguely ironic, giving the guy's reaction is kinda along that way (and the girl's discussion, too). I'd say the author nailed it. ^^