There's another reason free space is lower than expected: all filesystems use some space for 'bookkeeping'. In most Unix/Linux filesystems main consumer of space are 'inodes' ('index nodes', roughly equivalent to 'file headers' on DEC RSX-11 or VAX/VMS), the structures that store file attributes and physical location (not much more than filename is stored in directory entry).
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Dragi "Bonzi" Raos Q#5196 MkII #080000376, 18GB green MkIIa #040103247, 60GB blue