Yes it's theory, a very detailed theory looking at thousands and thousands of shots, providing a likelihood that they will result in a goal, based on thousands and thousands of goals and missed chances. It takes all variables out regarding the quality of teams, how defensive or attacking sides are, how many chances they give up, areas on the pitch they concede chances etc.This is all theory though. Expected goals and all that isn't fact.
Fact is we have let in 33 goals which is 3rd best in the league and De Gea is only 3 away from top spot clean sheets with Liverpool's keeper in a better team.
Based on how De Gea's performed this season (and he was worse last season), 11 of the other 19 sides would have conceded more goals had De Gea faced the shots their keepers faced.
Oh and Alisson's missed a quarter of the season and played in a far more attacking side and still kept 30% more clean sheets than De Gea
![Stick Out Tongue :p :p](/styles/default/xenforo/vbSmilies/Normal/tongue.gif)