There's also a difference between fault, responsibility, and ability to do something. They're interrelated, but they're not equivalent. If A is able to move 100 pounds, but not obligated to do so and not trying then A is able, but not responsible and not at fault. If A can only lift 20 pounds, works as a mover and gives their best effort then A is unable to move the couch, responsible for moving the couch, but not at fault as they've done everything they can do to move the couch. I could go on but my point stands: it's a weak, reductive metaphor.