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: Brentford was the better side by chances too.
Brentford: +0.6 for the season (4 in the table). Chelsea: +0.9 (10). The gap between points and xPTS is built from matches like this one.