Forum:Random Achievement?

Hey all, last night i was killing megaton settlers with my Silenced 10mm, Hunting Rifle and DEathclw Gauntlet.I killed a Sttler and i got the Achievement for the quest You Gotta Shoot 'em in the Head. I have never found the Repulic of Dave, Dukov's place or anything that has to do with that quest.Any idea how i completed the quest and got the achievement?

Bug or glitch.