df size mismatch

[a.shterenlikht at manchester.ac.uk]

why does df show that used+available < size?
in my case there is 20M missing for smaller partitions and
6GB missing for the larger one. How much can I really use?

anton

pw29# df -m
Filesystem  1M-blocks Used Avail Capacity  Mounted on
/dev/ad0s1a       247  129    98    57%    /
devfs               0    0     0   100%    /dev
/dev/ad0s1e       247    0   227     0%    /tmp
/dev/ad0s1f     27172 1453 23544     6%    /usr
/dev/ad0s1d       247    5   222     3%    /var