Based on this answer: https://emacs.stackexchange.com/questions/32377/selecting-correct-font-weight-variant-linux# I've found that by using "weight=book" I can make Emacs choose the regular variant. Still, it's odd that Emacs would choose the medium one when no such specification was given, especially since that's inconsistent with fontconfig (even if Emacs doesn't use it, I'd expect similar behavior at least).