What is the story behind Unreal's tilde key?

I was perusing the Unreal Input system because I recently wrote something similar for my custom engine, and I was looking for potential improvements/additions I could make, when I came across this comment on line 646 of InputCoreTypes.cpp:

AddKey(FKeyDetails(EKeys::Tilde, FText::FromString("`"))); // Yes this is not actually a tilde, it is a long, sad, and old story

This cracked me up, but then I had to know - what's the long, sad, story? The only thing I can think of is something to do with foreign language keyboards or alternate keyboard layouts, or something along those lines, but that doesn't seem particularly long or sad to me. Does anyone know what the deal is?