Thanks for the picture, 2mm does seem rather a lot I agree. I suppose these macro rails generally tend to be used more or less horizontally, could it be shimmed out at all? In principle macro rails with a lead screw offer much finer adjustment as opposed to those with side adjustment on a rack and will not tend to creep when held vertically. On the other hand for vertical slide scanning we only need a very small range of adjustment because the copy stand will get the camera to more or less to the right position so someone with some engineering skills and a milling machine could make something much more precise and without any play.
Still it is quite common to have to adjust for this with vertical copying systems, I have a separate baseboard underneath my negative holder and get away with a couple of card shims (set up initially with a mirror) but Mark D Segal uses threaded bolts at each corner and @gihlsong uses these incredible vernier adjusters.