Most hard drives do have "reserverd space"... but it is NOT acessable by the user, root or otherwise, ever.
What happens is this. All hard drives have weak or bad sectors. The onboard controller keeps track of sectors that are known to be bad.. and has a pool of extra sectors that are out of circulation from the factory. When it detects a bad sector, it automatically marks that sectors as bad so your computer doesnt use it, and puts a "spare" into use to replace it. This way, drives can be made cheaper, because they have a slight tolerance to being junk. Quite alot of sectors can go bad before you will actually notice problems with the drive. This is all done at a hardware level, on the actual hard drive controller... so the rest of your system/os has nothing to do with it. Unfortunatly, its not a fool proof system.. it usually works well enough with fairly healthy drives, but if its junk, its still junk.
I honestly dont know if some drives may actually go as far as to "shrink" beond their normal capacity, but I guess its possible. Most disk sizes are more of close guesses than exact estimates, because the manufacture never knows how many bad sectors a drive will have before its in a machine and running. Ive seen drives (once in awhile) post as larger than their real capacity because of this extra room... but usually they are a couple gigs smaller.
In a nutshell.... as long as your system is running properly, dont worry about it