FYI, I had a friend who made the same assumptions you made. He found, after four years or so with his boat, that not only was his alternator not working properly, but his batteries were not being charged.
All it would have taken, was, as we'd suggested, that he trace his wires from the batteries to the switch.
He finally did. Batteries and a factory (?) mis-wired battery selector switch
Really, don't take it personally, but "call a spade a spade" usually works.
As far as the positions and posts on the switch, many of us have had the very same misunderstanding. Me, too.