Because a breeder sells a proven breeder does not mean the animal is inferior.* Provens being sold are on average less productive producers. Some but not all are probably passed on because they are inferior to others the breeders are using. Maybe they lay more slugs or smaller clutches on average.
You have to understand that as a breeder your space is limited and there is a constant need to upgrade your collection year after year, so single gene females replace normals, 2 genes replace single genes etc.
Even if you buy an hatchling there is no guarantee that the animal will breed and if it does it won't be slugs![]()