There's space used by the inode table and other filesystem metadata. At the default nbpi (number of bytes per inode) of 4096, the inodes alone, at 128 bytes each, account for a little over three percent of the space.
If you increase the nbpi at filesystem creation, it should reduce the inode overhead, but it will also reduce the number of inodes on the filesystem, which will limit the number of files and directories you can create. Once all the inodes have been used, it doesn't matter how much space is available, no new files can be created until an existing one is deleted.
Some operating systems hide this information, but in reality they all have disk space dedicated to the actual structure of the filesystem.
Rod Knowlton
IBM Certified Advanced Technical Expert pSeries and AIX 5L
CompTIA Linux+
CompTIA Security+