xGA relies on volume of shot closer to goals with lesser blockers.
From this stats Man City GKs is underperforming because they conceded together 13 goals when their xGA is 12.04!! Similarly is Arsenal, Newcastle, Liverpool who are ranked team with lowest xGA.
We're ranked 8th with highest xGA meaning we allowed plenty of shots, and shots nearer to goals, with less blockers. The Brentford goal was not that high in term of xGA value, yet it was a goal due to Onana GK error. This wouldn't really showing in bad light in this type of stats because Everton chances like the one Mainoo cleared off the line has bigger xGA value and the goal was prevented for example.
https://understat.com/league/EPL