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: Lazio was the better side by chances too.
Lazio: +5.0 for the season (3 in the table). Genoa: −3.6 (19). The gap between points and xPTS is built from matches like this one.