I read somewhere that warmer incubators will make them hatch earlier.
Anyway, I usually have one or two that hatch earlier than the others, and almost always overnight between day 19 and 20. Right now, I have a bunch of eggs on lockdown in the hatcher. Day 21 is the 11th. I woke up this morning to find one chick already out, dried off, bright-eyed and bushy-tailed. Now it is 5 hours later, and only a couple of the rest are pipping.