Fever is not 100F. A fever is defined as 100.4F. Why 100.4 when 100 is a much easier to remember and handle number? Because fever is defined in humans as 38C, and that converts to 100.4F.
It's been a while but I think they tried to establish 100F as the average human body temperature. But after they established that baseline turns out they were off by 1.4 degrees and couldn't change it.
You're right. April 8th 2000 Christopher Walken caught a fever that changed the course of history forever. He had a fever and the only cure was more cow bell.
That's a sigfig error. A fever is 38C, which is 2 significant digits. Converting to 100° F goes up an order of magnitude so you get a free sigfig, but unless the original number was 38.0C, you don't get that 0.4, you're implying precision that the original measurement never gave you.
Well, if we go by SI guidelines, we could use m°C (milliCelsius) or c°C (centiCelsius) to have the precision of a thousand of a Celsius or a hundredth of a Celsius respectively.
But the fever definition wasn't that precise. They took the average temperature, 36.88 C, rounded it up to 37 C, and somewhat arbitrarily defined a fever as 1 C above the (rounded) average. Which is perfectly fine, but it means the equivalent in Fahrenheit is 100, not 100.4.