The old one will be slighty more compressed. I always fit new ones together. same height then
not sure if this is true but i think i heard that you can buy top mounts that do make the car lower because they are smaller? could the car have been fitted with these before you owned the car? http://www.venommotorsport.com/product_details.php?id=5556&manufacturer=JOM&category=Suspensionfor example
Have just digged out an old mount I replaced on my 8v. Looks to be about 45mm tall, I'm guessing it's been on there for the full 90-92k the car had done at that point.