From that photo I would actually say that the box is not a perfectly flat cuboid. If you look at the bottom left of the picture it looks to me as though the "face" is actually not perfectly straight. If you looked at this from the side I would expect it to look more like this:

     --- 
       |
       |
      /   <--- 
     |

The arrow shows our vantage point.

This doesn't seem to be entirely straight across the entire face either (unlike my crude ASCII "drawing"). Almost like a huge ball 'dented' the stone.

There's also a huge flash and glare going on which doesn't help and probably overexposes the specks that you see are also present in other pictures (like you posted in the sibling thread).