http://www.youtube.com/v/9O9HfafzBPE?version=3&hl=ru_RU (http://www.youtube.com/v/9O9HfafzBPE?version=3&hl=ru_RU)
I think it comes down to the fact that we are led to think that the end result is the same each time the tiles are reset. The tiles are actually a little loose in the case so allowing a marginal difference in the size of the oblong created. If you measured the oblong each time it would be different. Still very clever and make you think!