The spacers aren't meant to be tight on the rib due to rib tolerances. They are simply so that the mount isn't put on crooked. There will likely be .010-.015 play max in some guns, but some will be tight. Once it is clamped it won't move. The slot is not even needed once it's clamped, it could be a flat plate. It won't move.
I've even seen a 6mm rib which should use the #1 spacer have to use the #3 due to the cerakote. There's only about a .030" difference between #1 and #3.