It's subjective because it accounts for stolen bases and the catchers ability to stop people from running.
That's illogical, especially for Posey, because he catchers a lot of pitchers that absolutely suck at holding runners. It's not Posey's fault the pitchers have terrible moves to 1st, and a elongated deliveries with guys on 1st or 2nd.
Ask any SF fan, base stealers get tremdous jumps on their pitchers routinely.
Not fair to Posey to ding him for stuff out of his control.