Different flocks have their own characteristics. They may be purebreds, but they are a strain of that breed with their own traits and characterisitcs. With chickens, unless you carefully select for certain characteristics each generation, your flock will quickly lose those specific traits. Chickens have a lot of different traits you can select for: how fast they mature, color, patterns, size, conformation, skin color, number of points on a comb, egg laying frequency, egg laying longevity, age to start laying, broodiness, fertility, size of eggs, color of eggs, feed to egg conversion rate, feed to weight gain conversion rate, resistance to certain diseases, and many, many more. The traits any flock of chickens depends on the traits the person selecting the breeds uses as criteria, his breeding methods, his ability, and several other things.
Some breeders carefully select one specific rooster to pair with one or two specific hens to tightly control what the offspring might look like, and they still have more that don't quite meet their standard than that do, but this is about as consistent as you can get. Others, hatcheries for example, use a pen breeding method to mass produce chickens, keeping maybe 20 roosters with 200 hens and having no control over which rooster mates with which hen. You really do get inconsistency here. The one you mentioned, with 1 rooster with 15 hens, will probably fall somewhere in between the first and second example in consistency, depending on how good he is at selecting the hens and one rooster and what his criteria is. But that consistency also depends on what traits he is breeding to be consistent.
There are often hidden undercurrents in the flocks traits too. In a hatchery situation where the hatcheries select their breeders from their flocks, the hens that start laying earlier (early maturity) and that lay more eggs tend to get more of their offspring selected to the breeding flock. So after a few generations better egg laying and earlier maturity may become a characteristic of that hatchery breeding flock. If someone is selecting for other specific traits, early maturity or egg laying ability may not enter into the picture.
There may also be some differences in how they are fed that you don't quite see. For instance, do either of you feed different treats in different quantities? Is the quality of forage the same? If she is letting them forage on a manicured lawn where all they get is grass, while yours get different grasses and weeds, grass and weed seeds, and lots of various creepy crawly, flying, hopping things, yours might actually be getting a better diet.
Hatchery birds meet the requirements for a whole lot of us, and even that depends on which hatchery you get them from. Some people want something else in their chickens. They need to find a breeder that knows what they are doing and is trying to breed to the characteristics that the person wants.
So I agree with the others. There is no real surprise that hatchery chickens may grow faster ad mature younger than a specific breeder's chickens, but the breeder's chickens may wind up bigger at full maturity. It just depends on the flock characterisitics.
