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.
Lecce won against the run of chances: Monza created more (3.09 xG against 1.95).
Lecce: +3.8 for the season (12 in the table). Monza: −4.7 (16). The gap between points and xPTS is built from matches like this one.