Hmm, I have no idea then. Maybe its just the genetics of that particular breeders lines.
I get the occasional tiny chick that stays tiny. I have one in my coop now. She is about 6 months old now and half the size of a couple of cockerels I kept back that are 2 months younger! She was hatched by a broody, and her mom is still mothering her (its pretty darn cute). She's active and healthy as can be, just very, very small. I actually had 3 chicks in that hatch, and they were all small/seemed behind, I only kept the one, so I'm not sure if the other 2 ever caught up or not.
Anyway, just rambling. If yours seem healthy, I wouldn't worry too much about them. Just keep an eye on them