Ah. This all originates with function. A Fluffy corgi's fur is fine, mats easily, and picks up dirt and debris easily when outside. It also very easily collects ice balls and does not repel water, like a regular corgi's coat. This would make it very difficult for a Fluffy to effectively herd. So that's why they put it on the standard as a serious fault, and responsible breeders are trying to breed it out.
Although, I am glad there are accidental Fluffies, because I just love them. >^_^<
) Thank you for the information.