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.
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.
