I was a first time hatcher 2 weeks ago too. My chicken eggs hatched on day 18. Fortunately, I had just ceased the rotation and put in the hatching mat but they simply weren't reading the rules right!
One thing I noticed is that I overfilled the external reservoir and the eggs had max humidity from about the start. Still, of 7 eggs I got 5 chicks and early or not, they're all healthy at 2 weeks. They also all hatched with 24 hours. The ones that didn't hatch got 5 more days but just couldn't finish the deed. Maybe they needed more rotation until the time was better for them....
Also interesting to me is that they all had a single rooster but at least 3 different breed hens: a Cream Legbar, an Orpington and a Wyandotte (at least). Maybe a Plymouth Rock as well. Nevertheless, however different in size and other characteristics the hens are, the chicks are about as identical as they could be at this point.