Well I think this "number 1 reason" is a bit subjective then. From what its worth:Like I say, even if you ignore that aspect we are still one of the worst in the league for converting our chances. That is reason number 1 why we don't score enough.
Liverpool
xG 74
G 67
Manchester United
xG 58
G 49
The "underperforming" the xG scale is close, the xG number is higher so the final number of goals is higher as well.
Granted: I guess todays match isn't included in our totals, the match in hand obviously isn't as well.