Keep an eye on your Elo not rank, rank isnt always in sync with your Elo.
Your Elo will have gone up and his Elo will have gone down, but the server may have checked and the person above him in "rank" had less Elo than him (maybe they lost a game and lost Elo, or he won some games before playing you and it hadnt refreshed yet) so the person you played went up.
The opposite may have been true for you