@James-Menhenitt Think of xG less as "expected goals" and more like "expectation of a goal". It's basically the probability that a given shot made by the average player will result in a goal.
A value is given to each shot based on tons of factors, including distance and angle to the goal, type of shot (header, volley, etc), position of the keeper, the type of pass that led to the shot, etc. The xG values for each shot are then added together to give an xG value for the whole match.
The Overall > Average > xG in the software then takes the last 10 home matches for the home team and the same for the away team, and gives you the average xG figure per match over that match history.