Factors considered based on 300,000 observations:
- Distance from goal
- Angle of the shot
- Did the chance fall at the player's feet or was it a header?
- Was it a one on one?
- What was the assist like? (eg long ball, cross, through ball, pull-back)
- In what passage of play did it happen? (eg open play, direct free-kick, corner kick)
- Has the player just beaten an opponent?
- Is it a rebound?
By definition, a top 4 team (and particularly title winning teams) will have beat the xGs and xGa then, due to better individual players? Just means your players are better and are on form? If you are trending close to average you'll be midtable and if you're below average you'll be towards to bottom end of the table.
It'll be interesting to see how many times title winning teams beat this number.
Even then, depending on playing style, you may find some teams have a normal xG (scored and conceded), but much better xA if there is a stat like that (if you take into account the chance of beating a press, a tackle and creating a chance from midfield). They are just fancy ways of saying a team is better.
Of course, as with any stat if it happens in one game it can be an anomaly. If it happens over 17 (half a league season almost) games, may be there is a different inference.