You are correct, I was confused. Sorry.
There are different reasons eggs can hatch on different days. Heredity, humidity, how and how long they are stored before incubation starts, and just basic differences in the eggs can affect that. One big factor is average incubating temperature. If it is low they can be late, if it is high they can be early. So it might be for the entire hatch, it might be for an individual egg.
Each hatch can be different. One incubator hatch I had one chick hatch late on Day 19. Nothing happened (pip or anything) until late on Day, 20, just before I went to bed. Then a couple pipped. When I woke up the next morning an additional 16 had hatched and the hatch was over.
I've had several incubator hatches where all eggs that were going to hatch had hatched 16 hours after the first one. I've had broody hens take their chicks off of the nest less than 24 hours after the first one hatched.
I had one broody hen that hatched a chick late on a Monday night. I don't know when her other chicks hatched but she did not bring them off of the nest until early Friday morning, about 80 hours later. All of the chicks were fine and I did not provide any food or water for them while on the nest.
I generally don't have hard and fast rules where I always do things by a deadline. I try to assess each situation on its own and make a judgment based on what I see. If a broody brings her chicks off of the nest on Day 21, that hatch is over. If she waits until later, I wait. I've only once had a broody hen not hatch a few chicks. An egg broke in the nest and got raw egg on the other eggs. All of the eggs became rotten so it was obvious none would hatch. In an incubator I leave the chicks in until I'm sure the hatch is over, then remove them. Then I open the unhatched eggs to see what happened to them after I take care of the baby chicks.
I can't tell you how I know when the incubator hatch is over. I pay attention to what has happened and at some point based on history I end it.