”The SM-50 Micrometer shortens when you screw it“
This is only an optical illusion.
The stage is bigger than the micrometer-screw and you think automatically that the stage should be stable and screw has to move.
You know, there are no absolute positions in time and space :-)
So, it depends on that which part of the screw is mounted stable and cannot change its positon and which part is moveable.
If the part of the micrometer-screw with the scale cannot be moved then the second part of the screw has to move (it has no choice :-)
Please take a look at the normal screw:
And the same thing with the micrometer-screw (blue parts are stable):