I hate sharpening knives on sandpaper. I would rather use a stone or belt sander and then stone, much quicker for me!
Yeah, plus sandpaper costs more, over time. This is not an expensive stone.
Taking a knife through a 4 stage sandpaper progression would drive me nuts.
As far as why this happens, I do not know. It might be something to do with wear resistance, but you can work a knife all day on this thing and it still leaves a terrible edge. I wish I had a microscope at times like these.