The amount of people who wear size 20 shoes (about size 56, for you Europeans) is a pretty small group. Most size conversion charts don't even go up that high. The only person that we know with huge feet, Shaquille O'Neal, had size 22 feet, but he's one of a very small club. In fact, the person with the biggest feet in the UK has only size 21.
We're going to do some pretty loose math here and say there's about 100 people in the world with size 20 feet. Which makes the odds of one appearing in a shop pretty slim indeed. Not only that, but it makes them appearing in a shop which doesn't even sell shoes just about close to impossible. But guess what? That's exactly what happened.