Peeing on the edges of the litter tray

Hi, my cat pees in his litter tray but he goes on the sides, so obviously it sometimes leaks onto the floor too, but mainly the edges are always covered in wee when I lift the lid: I have to give the sides a wipe every day to clean them. I scoop it out every day and do a full empty once a week. I don't understand why his pee hits the sides of the tray? Does anyone know why? Thanks in advance!