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: Rennes was the better side by chances too.
Rennes: +2.5 for the season (5 in the table). Le Mans: +0.8 (9). The gap between points and xPTS is built from matches like this one.