Yeah, but the data is drawn from aggregating shot success across the Top 5 Leagues. So for every Harry Kane you have a Vincent Janssen.
The short answer is that better players will (in general) exceed their xG more, but that even they don't do that consistently and are just as likely to undershoot xG as to overshoot it. So a team undershooting their xG may have a problem with finishing chances at that point in time, but unless there's a player-specific problem you'd expect it to even our (as per the law of averages) over a season.
From that we can see that Spurs' good run is at the moment contingent on Son finishing every chance he gets, as Kane is only shooting at his xG. If Son hits a bad run then Spurs will immediately start dropping points as they don't have the creativity margin of chances created to afford to miss those.