I think I have it. The bottom of the drawer is co-plainer with the drawer fronts. And if you look at the ends of the drawers near the center you can see they are the same diameter as the inside of the drawer side. So a few caster facing up support the drawer and several caster running on the drawer inside diameter keep the drawer centered on the rotation.
Why the outside diameter isn't the same as the outside ends of the drawer fronts? Perhaps to form the impression this is an ordinary drawer??