Thats not uncommon. Sometimes breeders will breed a hairy cat back into their lines to change up the blood lines.
I did some reading on this condition, I cant seem to find anything on the Sphynx breed but polydactylism runs mainly in main coons and another breed. It could be possible a Main Coon "non" polydactyl was bred into the lines who had a parent who was polydactyl "positive" and that MainCoons babys would have a 50% chance of being polydactyl.
Polydactylism is quite prevalent in the free-roaming foundation stock in New England which was used to develope Maine Coons. However, it was cat show judges who put an end to it as they said that they wouldn't approve Maine Coons for advancement to a recognized breed if their breed standard included approval for polydactylism. So polys became a disqualification and the trait was bred out.
There are some breeders who are working with polys (and have been for years) but polys are no closer to acceptance than they originally were. Not to mention that there's a big division between poly breeders (who knowingly breed in a show fault) and other breeders such as MCBFA breeders, who agree to not knowingly breed faulty cats.