Try it yourself. Push out a fresh load, then stand up to wipe your behind. Wipe and wipe until nothing is coming off and your paper is staying white (or whatever colour your paper happens to be). Now sit back on the pot, get a fresh piece and wipe again. You'll be amazed!
Standing up actually causes your cheeks to clench together, causing some of the excrement to get "lost" in the crack. Sitting down is a much cleaner operation.