Probably a combination of your opponent's defensive setting, tempo choice, and a favorable battle roll for you (or just consistently good luck for the duration of the game, if you don't want to believe in such a thing).
I see as many as a handful of these wacky sims on every team, every season, to be honest. Not all *exactly* this extreme, but pretty close. The rebounding here is especially telling. Probably exaggerated because of fatigue (tempo choice). But still goofy. As you noted, you do have a solid rebounding advantage, and should have won that matchup decisively, but 21 is an extreme figure. It's not like they were all in the yellow-red the whole game. I would have considered +8 to be about the upper limit of normal for you in rebounding and you obviously destroyed that here.