Well about 7.3% of your hard drive will be lost due to formatting. The 3GB could be made up of hidden files. OR some computers have a hidden partition which has a certain amount of your hard drive in it. I think windows can recognise this partition but does not count it for some reason.
actually, it might not be 7.3%. it is determined by how many clusters there are on the hard drive. not all hard drives have the same amount of clusters.
1 cluster = 1 address on the File Allocation Table. the more clusters, the more addresses the File Allocation Table lists, therefore the more space on the hard drive is used.
and the only hidden partitions are made by Windows 2000 and XP, which take up 8MB. there is one hidden partition per drive, and only when they're formatted to NTFS.
killian explained the drive math pretty well though, which accounts for most of what is lost on any given drive. the File Allocation Table uses up little space.