I might have made some mistakes here, so I'm prepared to be corrected if anyone knows better. I don't know about larger concerns or battery farming, but some of the eggs ARE fertilised. This is why a cockerel is kept with them (and also why I always pick out anything floating in an egg). However, when the hens are laying eggs, they always lay the same number of eggs (five I think). When you collect the eggs, you leave one behind, so the hen then lays more to replace the ones you've taken. As they get old they stop laying, which tends to be when they go to the big hen house in the sky.