I don’t know where you got the birds you are comparing them to. Each individual hatchery has its own people that determine which chickens get to breed. They all have their own criteria they use and their own skill level. I’ve had the same breed from different hatcheries. There can be a lot of difference between hatcheries.
The same thing can happen between breeders. Some breed for different goals than others and they each have their own skills. If one is breeding for show and know what they are doing, their birds will be bigger than hatchery stock. If their goals are production oriented, maybe not so much difference in size. Or you might get a “breeder” that gets hatchery stock, knows nothing about the SOP, and sells “purebred” birds.
Unless the person selecting which birds get to reproduce pays attention to certain traits, chickens can quickly lose those traits. That’s not just size but includes about any trait, egg shell color, eye color, comb size and appearance, body shape, behavior traits, whether hens tend to go broody, and many many more.
While some hatcheries are better than others, don’t expect hatchery birds to hit the SOP requirements that closely. Their breeding methods are intended to mass produce chickens that somewhat follow the SOP but not hit them nearly as well as well as a qualified breeder that selects which rooster gets to mate with which hen. Their prices reflect that difference too.