Sounds like it is normal, they do move up and down about 25 - 30mm, they have mounts that are what's called rose joints which are basically a ball bearing with a hole drilled through the centre to accommodate the mounting bolt.
This set up allows the damper to be able to angle itself correctly for the damper rod to move in and out of the body unrestricted.
If you're concerned over the movement at all then best to get it checked out by your local friendly technician.