OK, sounds like something I saw on "Embarrassing Bodies". You know how bowel can get diverticulum, little pouches or bulges in the wall, the same thing can happen in the anus. So you expel bowel contents, but little bits of faeces are caught in the pockets rather than coming out with the rest of the stool. Which means it may get dislodged with the pressure of wiping, but the other problem is it comes out later with gravity (this guy swore he wiped forever and then hated fact his underwear then was dirty) I am not sure of the term, I'll come back if I can find it.