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.
Atalanta created more (1.48 xG against 1.31) and left with nothing.
Atalanta: +3.3 for the season (11 in the table). Cagliari: +6.2 (4). The gap between points and xPTS is built from matches like this one.