It’s not just temperature but how long they are at that temperature. What you are measuring is air temperature, it takes a while for the denser egg to raise the temperature inside to match. So a short spike isn’t always deadly, even a pretty high spike. The core temperature of the egg doesn’t change really fast.
People want to believe in absolutes, in this case that at a certain temperature everything is great but let the temperature raise (or drop) just one tenth of a degree more and it’s instantly fatal. It doesn’t work that way. It’s a gradual thing. Different eggs can handle different temperatures. As the internal temperatures rise past a certain point chicks start to die. Your hatch rate starts to drop. You may still get some chicks to hatch, just not as many.
It’s not a specific temperature, it’s a range. And how long the air temperature is at that spike makes a big difference. If you have to have an absolute that caused universal death, I’ve seen a sustained 103 degrees given, but I have no idea where that magic number came from. I’d much rather have a short spike to 103 than a sustained 102.