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.
Alavés created more (1.31 xG against 0.84) and left with nothing.
Alavés: −0.5 for the season (6 in the table). Valencia: −1.9 (19). The gap between points and xPTS is built from matches like this one.