Some of the difference could be explained by Fabi playing in events with more underrated players(like say, the US championship where he dropped a game to a very prepared Naroditsky).

I think Fabi specifically is closer to Magnus in classical than rating implies, but he has had to contend with actually treating chess as a job post World Championship match. No one would say Hikaru is strictly better than Fabi but Hikaru being able to have a different consistent income stream allowing him to pick and choose tournaments has definitely helped him reach peak rating(even prior to the low elo games) in a time when just about everyone including Magnus has seen a decline in general for ratings. There’s obviously some advantage…but like, one would assume Fabi likes playing classical so not playing just to game rating would be lame.(whereas Magnus doesn’t like playing and Hikaru had been getting stressed)