To be fair, it can happen. If a job is covered last minute or split up between spare drivers, given to another depot to cross-cover etc, those making up the diagrams have been known to omit important details such as bringing one out to the platform.
I've been given a job before where my cobbled-together diagram says I take a train over in the platform only for there to be no train, nobody claiming any knowledge of it's whereabouts and a long delay before someone discovered it was sitting in the siding, telling me this information and asking me if I wouldn't mind walking down and bringing it out. The idea of it not being brought out into the platform because it wasn't on anyone's diagram is not far fetched.