The age of the embryo when the egg is laid varies. The rate of development of the embryo once the egg is laid also varies. If the eggs are cooled after laying, development of the embryo ceases until the temperature rises again.
Development can resume even after the eggs have been cooled for several days.
The temperature of course will vary when the hen leaves the eggs to eat, etc. All of this affects the rate at which the embryo develops and, therefore, how long it takes for the chick to hatch. At this temperature the eggs will hatch in 21 days.