I don't think they hatch early except certain breeds, like Seramas? Are you hatching Seramas? Only some people have reported early hatches on those. If your incubator was running on the warm side, but not warm enough to kill them, they could have developed faster, if cooler, but not cool enough to hurt them, they'll hatch later. It's recommended that you wait at least until day 24, but they almost always hatch day 21 like clockwork!