Xgf would be calculated by the average chance of a goal from that spot on the ice. So no, it wouldn't take into account individual sh skill but instead uses league averages. Auston Matthews will out perform his xgf% and Cozens underperform. The percentage comes from taking shots given v. Shots taken. So Buffalo is giving up slightly better chances than they get as a team... considering Cozens line is terrible, that's about right.