It’s just a fact that online ratings are inflated. Hikaru Nakamura is rated 3231 (at the moment I write this) in blitz on chess.com but 2874 in blitz over the board, according to FIDE. That’s a difference of 357 rating points!

Of course, Hikaru has admitted that he deliberately works to inflate his online rating and has talked about the differences in the rules/mechanics online. For him, the lack of an increment in his online games (FIDE over the board blitz has an increment) and his well-practiced mouse skills help him a lot. Hikaru can flag a lot of people from a losing position, so he gets many more wins than he would have over the board. He also says he deliberately “farms” lower rated players to boost his rating by a small amount, and avoids games against dangerous opponents who are underrated (due to a lack of online play).