The key here is incubation -which does not start until the hen stops laying and starts her 24/7 vigil over the eggs leaving the nest only briefly each day to eat, drink and eliminate. While actively laying a hen might spend an hour or so in the nest before laying an egg and a varying amount of time after the egg has been laid while she recovers - but she then leaves the nest and spends the majority of her 24 hour day NOT on the nest. Once a hen feels she has the right size clutch of eggs production of more eggs stops and she "sets" the nest, starting incubation and all the eggs hatch within a day or so of each other because development of the chick in each of those eggs starts at the same time.
