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.

Hasn't the fever temperature changed recently or something

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.

A fever is defined as 100.4F

Who defines it like that? I'm asking because I wouldn't be surprised if the definition differs between orgs

°F and °C, unless you're speaking of Coulomb and Farad.