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.
The score matched the game: Cagliari was the better side by chances too.
Cagliari: +6.2 for the season (4 in the table). Lecce: +3.8 (12). The gap between points and xPTS is built from matches like this one.