Without going into it too much, I currently have a theory that the game has a "New Game+" customization menu that allows you to alter various things for personal enjoyment/replayability, so while there's three core difficulties that change some aspects of the game and arrange them, I think there's more things you can change for each difficulty with the customization options (we have it confirmed that you can turn your "armor", aka how much damage you take, to be more or less on New Game+, as well as things like turn Infinite ammo on, there's various screen filters, and I believe other things they haven't revealed yet).