I think the answer you're looking for lies in 2 places. Home court advantage and enthusiasm.
He gets an edge that doesn't show up in the ratings playing you at home and when I look at enthusiasm, I can guarantee that he had a lot more saved in the bank. He played normal in the Quarter finals while you played crunch. And when punching your teams into basketball prospectus, he played a lot of TIE this season whereas it shows that you didn't. So his lesser players played at a higher level than yours. That shows in the boxscore where his stats were far better but not in the ratings. My guess is that your enthusiasm for the game was at 5 while he was at 8-9.