An egg does not have a day’s worth of development 2 seconds or 2 hours after you put it in the incubator. It takes 24 hours for it to have a day’s worth of development. Instead of thinking Day 1 or Day 18, I like to think of it as days of development. It’s just easier for me that way. An easy way to check your counting is that the day of the week you put them in is the day of the week they should hatch. If you start them Tuesday, they should hatch on a Tuesday.
That’s only theory though. Reality trumps theory every time in this. Like Kyzmette said, the average incubating temperature can have a big effect. If the average incubating temperature is a little high they can be early. If the average incubating temperature is low they can be late. But there are a lot of other things that affect that too, like heredity, humidity, and how and how long they are stored before you start them. There are just some basic differences in the eggs. Some egg whites are more watery than others. Those tend to take a little longer. Smaller eggs tend to hatch a little earlier than larger eggs.
All this is why a hatch may drag on for two or even three days. Most of my hatches are over within 18 hours of the first one hatching, but I’ve had some hatches drag out more than 48 hours. I’ve had eggs hatch more than 2 full days early in an incubator and under a broody, all within a few hours of each other. I’ve had eggs hatch on time in an incubator or under a broody. They are not consistent and each hatch is unique. It’s nice when they all hatch about on time and within a few hours of each other, but reality is not always nice.
There is often a big difference in things that might can possibly happen and something that will absolutely happen each and every 100% of the time without fail. Shrink-wrapping a chick by opening the incubator after an egg has pipped is one of those. In an egg has not pipped it is not a big deal, but if an egg has pipped it is possible you can shrink-wrap the chick. I’ve done that myself. But I’ve also opened the incubator and not shrink-wrapped a lot of chicks that had already pipped. I understand there is a risk if I open the incubator, but sometime something happens that it’s worth the risk to open it.