Its tough while incubating, we worry and fuss - me included, but sometimes a hen will set in a sunny place and she will get a bit warm too, and for a short amount of time, she leaves the nest to drink, eat or rest, and the eggs may be in the shade, so they cool down a bit.
Nature takes into account for all these things. But sometime nature is not on our side!
I'll be looking forward to hearing how your hatch went.
Oh, and by the way, more of us have had "failure" batches. We all get mad, but in the end, we can find another batch of eggs to tuck in there.
Good luck!