my assumption is that box + 1 formations (inside or outside box) allows for a man to be attached at all times to the best outside or inside threat, while the box sort of cycles around. In theory, this would lead to 2 players covering the main threat at given times. Similarly, a Zone may collapse on a given guard or forward once the ball is received.
Whether or not the game engine is smart enough to recognize this? who knows. it may only assign one defender in calculations.