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.
Venezia created more (1.85 xG against 1.77) and left with nothing.
Venezia: −5.5 for the season (20 in the table). Lazio: +5.0 (3). The gap between points and xPTS is built from matches like this one.