Having now read the story, it's just errm... internally inconsistent. And I don't mean that in the "functional" way Priest intends. When the box is first opened the statue is not treated as something that's both not there and not - instead, it's treated as an object that has property X, where X is "looking at this object causes a human to believe it's both there and not". This is not inconsistent - it's just a weird property of an object, which doesn't actually exist in real life. Then at the end, the world is split into two branches in an arbitrary way that doesn't follow from property X. Looking at it another way, "inconsistency" is very poorly defined and this lack of definition is hidden inside the magical effects that looking at the object has. (It would be clearer if, for example, he dropped a coin on the statue and then tried to pick it up - clearly the world would have to split right away, which is hidden in the story under the guise of being able to see property X.)