There are a lot of different reasons an egg can hatch that much earlier than the others, either under a broody hen or in an incubator. It's possible your incubator had a warm spot, but heredity, how and how long an egg was stored before incubation started, and just basic differences in individual eggs. maybe it was stored a bit warm so incubation actually started early for it. An egg will develop some in the 80's Fahrenheit, just not much and it can't sustain development.
I've had a chick hatch early like that before. Recently one chick hatched about two days early. No other egg even pipped for a full 24 hours, then when I woke up the next morning the remaining 16 were out of the shell. Each egg is unique, each hatch is unique.
Sometimes I have entire hatches two full days early, either under a broody or in the incubator. Occasionally they are right on time. With mine I think it is heredity. That 21 day thing is a general target, not something written in stone.
If you are getting good hatches I would not change a thing. If it ain't broke, don't fix it.