Yes, the assumption is that the eggs are fertile in the "Fertile Eggs" box. But, to tell you the honest truth, in talking to people they're amazed that stuff will hatch out of supermarket eggs and they've never seen "Fertile Eggs" at their supermarket. Many stores (not all, but especially organic grocers) sell fertile eggs. Many stores don't. So if you've never seen fertile eggs at the supermarket it would definitely seem very odd indeed that chickens can hatch out of store-bought eggs.
Now, interestingly enough, Trader Joe's was actually having complaints from folks about their fertile eggs not being fertile, i.e., no bullseye on the egg to indicate a chicken mating had taken place. In fact, I was kind of expecting only one or two chicks to hatch out per dozen as had been the history of me, incubation and TJ eggs in the past. So I set 3 dozen eggs earlier this month hoping to add a pullet or two to my coop...3 dozen eggs = 2 live eggs per dozen (or 6); 6 divided by 2 for boys/girls = *maybe* 3 pullets. In the past A LOT of the TJ eggs were duds. But it appears the Trader Joe's flock farms had recently upped their rooster potency game and shipping out very fertile eggs because I certainly wasn't expecting 26 chicks! (those 26 in my avatar)
