Out of curiosity I setup one of our DNS-320 boxes wit a couple of Samsung 1TB drives and formatted them as RAID 1.
For that I got: Total "Volume_1" Space = 981351 MBytes, which, if my maths is correct shows a 'missing' 18,649 MBytes and that compares very well with the same drives formatted as JFS on our IBM OS/2 workstations (if we use HPFS formatting we get a little more overhead used because we need a couple of partitions on that size disk).
There are several things that can cause 'strange' formatting capacities with most of them being down to windows monopoly position in the consumer market and how manufactures of computer parts react in the consumer market. By that I mean, in the case of disk drives, the firmware is set to assume that formatting will be windows NTFS or, as a worst case FAT32. The moment any other format is used strange capacities can be, and usually are, reported. This is not the case with enterprise drives for the simple reason that Linux has the larger share in that market.