Each hatchery has its own people deciding which birds get to breed so you can get differences from that. Each hatchery has its own business practices so there can be differences in what they do. But most follow a certain general practice. Most of the people selecting which chickens get to breed have a degree in poultry science, including genetics. They select which chickens make it to the breeding pens by looking at the SOP to a certain degree. They want their birds to represent the breed but they are not out to breed grand champion prize winners. They are out to sell mass market chicks at mass market prices. Most use the pen breeding method. That's where they might put 20 roosters in one pen with 200 hens. These mate randomly, they do not control which rooster mates with which hen. That's not how people that breed grand champion chickens work.
There is actually more difference in so-called "breeders" than there are in hatcheries. Each breeder has their own goals, which can vary. Some are more knowledgeable than others. Some breeders are out to win a grand prize, which is not easy by the way. They carefully breed certain roosters with one or two certain hens to give them the best chance of a champion. Nothing random about that breeding. Some even have different lines depending on whether they are trying for a male champion or a female champion. The same flock will not always produce both. Many are only looking at what the judge sees. The judge will not see whether a hen goes broody or not or what color of egg the hen lays so these breed qualities may not be very important to them.
You have a few breeders that breed to win grand prizes but also breed for the characteristics the judge doesn't see, Very few of these. If you go to a heritage site they will often list certain breeds as endangered even though a lot of people are breeding that breed for show. What they are talking about in these cases are the things a judge doesn't see.
You sometimes have breeders that don't worry that much about the SOP. They may be breeding for certain production traits like egg laying or meat. What difference does eye color make if you are breeding for eggs or meat? To some it will make a difference, not to others. Size matters more if you are breeding for meat than if you are breeding for eggs.
You go all the way to people that get chickens from hatcheries and never see an SOP. They still sell their birds as purebreds. Hatcheries often do a better job than some of these.
You have people that are a combination of all of these. Just because someone calls themselves a breeder doesn't mean a lot to me. You need to find someone that is breeding for what you want. If you are looking for a bird that fits the SOP you will not pay mass market prices for them. They don't operate in a way that they could afford that.
I have never gotten chicks from Towline so I can't comment on them from experience. But then you believe they may have gotten them from Towline. Doesn't sound like you are sure. Are you sure they didn't hatch those pullets themselves, maybe from hens from Towline? If so, how well did they control which rooster was the father, was it even a Buff Orp? To me there are a few unknowns there.
Do hatcheries mix in other breeds with their flocks. I don't own a hatchery or work at one so I can't speak from experience. It's possible some do, I try to never say never. But it doesn't make sense. If you bring in new blood to address genetic diversity issues, bring in more of the same breed from another source. If two flocks have been separated genetically for several generations that will have as much of an effect on genetic diversity as bringing in a totally different breed.
I don't know enough of the full backstory on those pullets to make an informed guess. There could be some questions on their breeding. If they were fed a relatively low protein diet, say they foraged for most of their feed instead of being fed a high-protein diet, they will not grow as large as a bird fed a showbird diet. Feathers may not be as thick either. A lot of a buff orps size is due to thick feathers. Some look like they really shrink during a heavy molt.
As far as going broody, if the hatchery or whoever is breeding them makes going broody a capital crime then broodiness can be bred out of them. That has happened to a lot of production breeds. Hatcheries are in the business of hatching eggs with an incubator, not with broody hens. If you are breeding for show you are probably using an incubator as well. A broody hen is not laying eggs so she is not producing. But she is eating and drinking, plus require special handling to lessen her disruption on the rest of the flock. Some hatcheries or breeders put up with it since it is supposed to be a breed trait, but some won't. It is a business to take care of their families after all.
