One of my neighbors has two incubators and say that one always hatches 1.5 days early and the other always 2 days late.
I guess that temp has a lot to do with it. 1 degree up or down for the entire incubation changes the timing because it affects the metabolic process.
The Serama eggs I have been hatching are notorious for their unpredictability. Sometimes they hatch as early as day 18, other times they hatch just like normal large fowl eggs at 21 days, and sometimes if the incubator shorts out for a period of time they can even come out a day late. It's been a challenge. I have learned to hatch them my own way - disregarding just about everything I learned from reading up on hatching eggs. :|