I get the feeling, there are 2 devs in the Golang team, that were like
"Every idiot knows that Acronyms should be all caps."
"Da fuck?! Wanna know what should be all caps?!
The sign on your mom's brothel, dickhead!"

And so they both tried to establish their style as a convention in the stdlib, and that's why half the the Acronyms are written in all-caps and the other half isn't.