To make the shelves fit better with the hardware.  If I had known of the movement and noise, I would have reconsidered making this purchase. My cat hovers low to the ground behaving as if she senses the shelves are undependable and could fall due to the effect of instability.The shelves are loose and rattle-  giving a sense of instability-(the play in the shelf board's movement along with the sound -makes it annoying/startling-there is too much play in the metal hardware that connects the flat shelves to the ladder posts.  i will have to stuff tape or cardboard- to prevent rattling sound as well as the movement.