The idea was that it was difficult to be sure if you had got the images overlapping exactly in the rangefinder, so shells could be fired at the wrong range, falling short or long. In WW2, ships often had false bows and sterns painted on for the same reason, one quick way of gauging the distance was to see how 'wide' the ship looked in the rangefinder, by making the ship look shorter, a quick guess made it appear further away than it really was, and the shells would pass harmlessly over to a point further away.
They even painted bow waves on. The false bow wave also made aiming a torpedo more difficult as the ship looked like it was sailing faster.