Single combs are recessive so both parents have to be carrying the recessive gene to produce offspring that have single combs. The parents may have had walnut combs as part of their phenotype, so the breeder/hatchery can't tell that the parents are carrying the recessive gene until the offspring hatch. It's common with hatchery birds as the hatcheries generally produce quantity, not quality. Single combed silkies will always pass down the single combed gene to their offspring so they should not be bred from. This is one of the problems of buying silkies (or any other breed for that matter) sight unseen from hatcheries as you are not able to choose birds that are true to type.