It's just familiarity. I use Firefox on macOS and I always think the fonts look fuzzy on Chrome when I'm occasionally forced to use it. I spent a few weeks on Arc (Chrome-based) earlier this year, got used to the fonts, and then they looked a bit weird when I came back to Firefox.
I'm surprised Mac users are noticing a difference though, I find the difference basically vanishes on really high res displays (like my QHD-ish panel on my Framework 13).