df oddity (to a newbie)

[Joshua Eckroth]

I installed a 80gig harddrive for /usr/home, but df has been consistently giving me weird numbers:

Filesystem  1K-blocks     Used    Avail Capacity  Mounted on
/dev/ad5s1d  75685352 24426308 45204216    35%    /usr/home

what's with Used + Avail != 1K-blocks?

-josh