I have noticed that chickens continue to grow for at least two years, I keep mine on grower a lot longer because I believe they need the extra protein to continue to grow as well as produce eggs, without enough they will start feather picking or egg eating sometimes, I always provide oyster shells for calcium as well as feeding back their shells. After I switched from a layer to an all flock I had my birds molt faster and resume laying quicker, so for me higher protein was warranted.
Back in the day when chicken feed was still just a bunch of stuff mixed together hens didn't start laying until 7-8 months, now they are starting at 4 months. And I practice the old way of adding scratch into their diet to bring down protein to put off laying a bit until they mature more, I don't want my hens starting at 4 months, so feed can have a big effect on laying from what I have seen.
I have also bought birds from a show breeder, they are terrible layers, late to start, low production, despite eating the same feed as the hatchery birds, so breeding behind the birds is also a factor.
In the end I would say it's about 50 percent breeding, and 50 percent feeding.
Keep in mind your comparing apples with oranges.
Most hatcheries "breed" for egg production and in doing so they cross other high production breeds like leghorns in to there stock.
If you order Rhode Island Reds from a hatchery they are nothing like the real breed because of there crossing and breed high production breeds into them.
Now if you want a chicken that looks like it could be a breed, produces lots of eggs, has a short production life and makes a poor dual propose animal then hatchery stock is the way to go. However if you want a real breed of chicken that makes a good dual animal and has a longer production life than breeder stock is the way to go.