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: Napoli was the better side by chances too.
Genoa: −3.6 for the season (19 in the table). Napoli: −1.8 (9). The gap between points and xPTS is built from matches like this one.