Seems to be a problem with the capsule collider. It adds about 4 pixels to the collision. If you reduce the radius then it works OK but you get side collision interpenetration with the mesh.
If your game is based on just a ball rolling (like super monkey ball) then then you could just switch out the capsule for a sphere collider.
As a last ditch resort you can put the ball static mesh at a -2 offset on the z plane.