You're correct that you can set the revshare to anything (defaults to 10%). But beyond that it gives literally 0 information, so I can only assume it's transation fees. With the revshare at 10%, I was only receiving ~60%. I tried searching for info on this, but the best I could find was that the payment providers may take a flat amount and/or a %age, and that there may be a flat fee which Itch takes from your first few payments. But I couldn't find anything that would add up to that big a difference.
Meanwhile, on Ko-Fi, without changing anything I get 90%. I'd honestly not be too bothered by getting less on Itch if I just knew where it was all going...
Anyway, glad you're enjoying the game :)