Like Hillaire said they are not purebreds. I imagine whoever originally bred them had to mix some other breed in to get the lavender color and hasn't bred all the non-Silkie traits back out. The black skin color is a dominant trait so to get that many with lighter skin colors some recessives have to be showing up. Recessives are hard to breed out of a flock.
The feathered feet gene is also dominant, but if it is paired with a recessive the feathering is not as pronounced. Again, the recessive is hard to breed out.
Each breeder chooses which traits they want to breed for. There are a lot of Silkie traits you need for show quality Silkies. The more traits you breed for the harder it is. When breeders are developing a new color of a breed they normally have to mix in other breeds to get those colors or patterns.
I know someone that is developing a new color/pattern of Ameraucana, hoping to some day get that recognized. She's part of a group of different breeders working together on that project. She was on her 15th generation when I last spoke to her. She was pretty happy with eye color, body conformation, comb, and leg color but still had a few other things to work on before they would meet the standard as Ameraucanas. She did not have color/pattern that far along, it was going to be one of the last things they perfected.
It's not easy being a breeder and stuff doesn't happen overnight.
