Wasn't he voted best keeper in last year's CL campaign?
According to
FBref his shot stopping this campaign is worse than all but one other goalkeeper.
Last season he was top of the list. None better. Which is mad. 7th best at stopping crosses too. Looks like he's got a terrible case of Manchester Uniteditis.
You meant the PSxG+/- ??
PSxG+/- is calculated by
PSxG-GA (Post Shot Expected Goal minus Goal Against)
"
Post-Shot Expected Goals minus Goals Allowed
Positive numbers suggest better luck or an above average ability to stop shots
PSxG is expected goals based on how likely the goalkeeper is to save the shot
Note: Does not include own goals
xG totals include penalty kicks, but do not include penalty shootouts (unless otherwise noted). "
In Onana case, it was taking PSxG of 17.6 minus the 11 conceded goal = 6.6 (highest). So the key to understand this still lies with Post-Shot Expected Goals, and how they collect and quantify this stats.
"
What is Post-Shot xG?
Post-Shot xG (PSxG) is calculated after a shot has been taken and
generally considers shot placement, and if the model includes it, goalkeeper positioning in assigning a value between 0 (off-target) and 1 to every shot.
Traditionally, PSxG's primary utility has been in assessing goalkeeper shot-stopping ability, although
newer models that include extra factors like shot velocity could provide further insight into finishing ability.
"
https://statsbomb.com/soccer-metrics/expected-goals-xg-explained/
It can be interpreted that a tame shot aimed at far corner, even though GK could comfortably routinely get down to catch can have higher PSxG value than a rocket knuckleball from distance fired straight at GK, whom is forced to parry it because it's too scary to try to catch such shot.
So without the models with shot velocity, it's not a yet a reliable stats. And those model is not ready available for public. It seems to be only accessed or privately collected by professionals. This is where scouts who carry out eye tests on players are still uttermostly important.