Doesn't seem like you are going by your other post.
'Should' have scored 3 or 4 more on top of the 7 goals they did score, 10 or 11 goals in one game of football - truly that performance must have been the pinnacle of the modern game if they should have scored that many over 90 minutes as you never see a score line like that, like ever... even the 7 they did score is an irregularity. How often have Liverpool, City, Chelsea or ourselves (when flying) pulled off score lines like that over the last few years? Off the top of my head nothing leaps out. Could it be that the chances that you think should be reflected as a 1 in xg in reality aren't actually that high which is he reason why we don't regularly see games with those sorts of scorelines?
Regardless you seem to accept earlier that shots against is a valid measure of how a defence is doing but the more refined Xg stat which attempts to sort the 30 yard hit and hopes from the gilt edged one on ones isn't- which doesn't really make sense as it shows how many big chances a defence us giving away, whether you agree with the metrics or not.