What birds they use may be a sort of "crapshoot" depending upon their source(s) of eggs.
As Illia indicated, Sex-Links or Production Reds are a fair bet, since even the natural, holistic, hippie type of people have enough brain cells to see the profit potential difference between a 3 egg/week laying hen and a 5 egg/week laying hen...That said, I doubt seriously that more than a small percentage of those eggs would be viable for hatching. Fertile eggs for eating (if properly cold-stored) have a "shelf life" of weeks, however eggs for hatching have a much more limited window, so unless those eggs made it into the market within less than 10 days of being laid, they would stand little chance of actually hatching, based upon my understanding...
There are always stories of eggs older than that hatching out healthy chicks, (after being exposed to inhospitable elements, etc...)but I would consider those anecdotal.
Lightfoote