I do not think there is a dog anywhere that shed as much as Frodo and he had the shortest stock coat I have every seen, but I digress.
Maybe the pup is a coat. Kind of looks like it to me, but I have never owned a coat and never owned a sable. In my first litter I had a pup that looked almost grey to the other pup's black. I thought Cujo was a "blue." I did not try to sell him. He was supposed to go to my sister, but my parents got him. His grey became black and he is certainly NOT blue.
While I do not breed for blues, I know that people do and sell the pups at an inflated rate because it is "rare." It is a really serious fault and reputable breeders used to cull their litter of blues, livers, and whites. My guess would be that the breeder couldn't tell. You can be experienced, having several litters with no coats, until you introduce the gene. So you could have bred six or seven litters and then bred one of your bitches to an outside dog, only to find some of the pups fluffyier than what you are used to. Since the boy wasn't a coat, and the owners did not mention coats in their line, you may think it is just a little different then how your dogs generally develop their coats.
It is possible that this was an experienced and reputable breeder who had her first litter with coats in it.
Again, not a big deal if you are not showing. But if you perfer the look of the shor stock coat than, maybe you can exchange the pup if you contact the breeder.