Most people aren't even aware there's a "real" (or rather, fake) minus sign. I didn't know until reading this thread. The hyphen is right next to the +/= key, so it's not unreasonable at all to think the _/- key is minus.
There's also option+space for and other goodies. I recently added option+x for "checkmark x" and option+v for "checkmark v" since I like dropping those around more often than I should...
Btw, if you haven't already, check out Karabiner on macOS for AutoHotkey-esque functionality.