Skeetaboat, it's because your bowel can have blockages in there of 'old' poo that doesn't come out, so the 'new' poo just makes it's way around it. Not everyone would have that much food in your bowel at any one time - some people with large diets or sluggish bowels wouldn't poo often enough to evacuate it all and don't forget just how long our intestines are if they were stretched out in a straight line.