I'm a very slow player in general tbh. And in the last few in-game months I spent a bunch of time in Mementos collecting Stamps and indirectly grinding while watching Twitch. By the end of the game, I was pretty overpowered and had way more cash than I could ever possibly need lol.
I can't check since I've already uninstalled the game but I think my in-game playtime is probably somewhere between yours and the one I posted above. Ever since I switched to a dual monitor setup, I'm constantly clicking out of games to go check or do something else. Since the simulation pauses completely when you click out of P5R, I imagine those 5-10mins break added up over time to create a significant discrepancy between the in-game playtime and Steam's.