The fluff's are consided sub standard and are not to be bred. I picked this little girl as she has and had a much better temperament than her brother. Other than conditioning with a product like Samme (silicon conditioner) and brushing I think the hair is actually less (shedding wise) with a fluff.
The fluffy gene is recessive to the normal coat gene, and it is actually quite common in corgis. Many corgis carry the recessive gene, especially the dogs with "show" type coats. Even though they are not specifically bred for, when the two genes come together you can have several puppies in the litter that are fluffy and they are very cute
Unless I'm mistaken, Fluffy is a standard Mendelian recessive: if both dam and sire are carriers (heterozygous), they'll have standard coats and each of their pups has a 1/4 chance at the Power of da Fluff.
There is now a DNA test for Fluffy(!).
If, as you say, many of the dogs with "show" type coats are carriers, then it's not 100% recessive, and this could explain the prevalence of Fluffy: it has been selected for, even though breeders seldom breed full fluffies.
Al had fluffy littermates, so he's got a 50% chance of being a Fluffy carrier; maybe that explains his mink-like coat...