New Polands and a genetic mystery

We have swapped out Wyandottes for a trio of Polands today, very nice they are too, although Mr Poland needs a hair cut...so that's a job for me tomorrow. Have cleaned out the brooders and moved the 3 week old chicks into a new one to make room for the new hatches over the weekend. , Do have a bit of a mystery though, look at the pictures below:

Mummy

Daddy

Baby

Another baby!











All I can put it down to is that one of the parents was bred from a Light Sussex x Buff Sussex..Anyone have any suggestions as to how we have a Buff Sussex chick from 2 Light Sussex parents, we have 4 chicks from this group and 2 from the last group and all the other 5 are White. Lets see what happens when the next few hatch this weekend?