The score says how it ended. xG says how it should have ended: it is the sum of the chances each shot was worth, not an opinion about the game.
Sassuolo created more (2.27 xG against 1.61) and left with nothing.
Monza: −4.7 for the season (16 in the table). Sassuolo: −0.6 (10). The gap between points and xPTS is built from matches like this one.