The stat is independent of those variables, meaning you have to take that into account. Good players outscore their XG, that's a sign of being a good player. Same idea for goalkeepers.
The practicalities are exactly that, I think. It's kinda like saying if every player in the league had x chance, how many times would it go in? Strikers outperform, defenders underperform.
I think the weird thing about xG is that the numbers don't agree with our instincts, but really even the best strikers in the league (Isak, Haaland, Salah) miss presentable chances every single game. Even if that's 1 chance missed, 1 chance scored (which is probably what shit-hot Isak is doing atm), you can easily see how fairly easy looking chances can be 0.5 xG.