Oh gosh, they can have things like less down on their bodies, rough navels, not absorbing the yolk, all kinds of things. Is this the first time you've used this bator? I would have went with something a bit less expensive!
They can take up to 48 hours to hatch after pipping the internal membrane though, so just seeing them move does not mean they will hatch early. Mine always hatch around day 19. I always have at least 1 hatch on day 18, and they are done by day 20. Mine did this ever since the first hatch I did, and it still does it. The first hatch I did had the temp fluctuating between 98 & 102, and i have fixed that. So the temp swing did not cause it. I have heard a swing in temp between 97 & 102 is fine, provided it doesn't stay at either extreme for too long.