Again McAvoy plays with Bergeron, a superior defensive center to Tyler Seguin or Joe Pavelski. That's an enormous advantage for McAvoy and I am positive explains a substantial part of the defensive statistical difference along with Heiskanen being a year and a half younger. There is very little between these players, some have a bit better offense and some a bit better defense.
He does play with Bergeron, but it doesn't explain why McAvoy is so good at what he does or his relative numbers. You can have two great players out on the same time and it doesn't mean one carries the other. I think you're missing the boat on just how more dominant McAvoy is in his own end than a lot of his peers in his age bracket, Heiskanen included, who's not in the Fox/Makar tier. Fox won the Norris last year and Makar will most likely be one of the three finalists. McAvoy was 5th last year and will most likely be again. Heiskanen didn't even get a vote last year. Voters can be stupid but they're not THAT stupid to not even toss someone a pity vote if they felt he was good enough.