Chicken breeds tend to be very confusing in general, at least for me!
As Kevin alluded to, IMHO the biggest confusion arises because, unlike dogs, chicken breed registries are not "closed". In dogs, as long as you breed a registered dog of one breed to a registered dog of another breed, then the offspring is automatically a purebred. For example, I can breed a terrible example of a doberman to another terrible example of a doberman, but as long as those dogs had purebred parents then their offspring would be considered purebred as well -- even if the puppy looks like a rottweiler. OTOH, if I breed a good example of a registered doberman to a registered rottweiler, their offspring will NOT be considered purebred no matter how much they may LOOK like dobermans. And even if I then breed one of those crossbred dogs to pure dobermans for five generations down the road, then they will STILL not be considered pure dobermans.
But in chickens, all bets are off. A bird is considered purebred as long as it meets the APA standard for that breed, especially if it produces offspring that also meet the standard. Cross breeding is done VERY frequently, in order to introduce new colors and such -- but few registries are kept, so the ancestry of the bird is much less important than its external appearance and the offspring it produces. It's a different world!
I hope this helps!