The short answer: Called Shot always works. "I hit him in the arm" only works if the GM feels like it.
The long answer: The fighter's attack could easily have gone like this:
Fighter: "Bah! 2 points of damage."
GM: "The automaton doesn't even seem to notice as your sword strikes sparks off its impregnable brass armour. "
Or even:
Fighter: "Ha! 9 points of damage."
GM: "Wow, yeah, the thick brass armor soaks up alot of the force from your blow, but that's a lot of damage. You've broken some kind of piston and there's oil leaking everywhere, but it's still strangling the wizard."
However, if the Ranger had been allowed a Called Shot, you'd get:
Ranger: "Bah! 2 points of damage."
GM: "The automaton doesn't even seem to notice as your arrow ricochets off its impregnable brass armour and clatters under a table somewhere."
Ranger: "Dude, Called Shot says the target drops anything it's holding, regardless of whether I deal any damage."
GM: "...Yeah, sorry, you're right. Let me try that again. Your arrow cuts through a pipe of some kind, and oil sprays everywhere. The automaton's grip loosens, and the wizard struggles free and falls to the floor, grasping for breath. The automaton doesn't seem much fazed, though."