does not understand "df -H"

[Tsu-Fan Cheng]

Hi,
   I have a pretty basic question: when I "df -H" my disk, the numbers
cant add up,


Filesystem     Size    Used   Avail Capacity  Mounted on
/dev/ad0c      484G    429G     17G    96%    /ad0c


there should be more disk space available based on what is shown. why
is this like that??

thanks!!

TFC