df -h reporting weird numbers (FreeBSD 4.8)

[Joel V]

/dev/ad0s1g   108G   5.2G    95G     5%    /usr
/dev/ad2s1e    55G   428M    50G     1%    /mnt/backups
 
Okey, 55GB minus 0.5GB should be 54.5GB according to my math. I wont
even mention /usr. Where are the missing gigabytes?
Btw, drive is 60GB WD special edition, works perfectly, checked the
drive with Data Lifeguard etc. Filesystem is UFS.
This is the entry in fstab:
 
/dev/ad0s1g             /usr            ufs     rw,userquota    2
2
/dev/ad2s1e             /mnt/backups    ufs     rw              2
3
 
 
 
Cheers.