output of df doesn't add up

[Micha Feigin <michf@post.tau.ac.il>]

For some reason the output for my main disk as given by df doesn't add up:

Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1             142G  123G   12G  92% /

142g - (123g + 12g) = 7g

it seems that I have 7gb missing on the disk (its ext4,  just crashed, I think
because it went to 100% and required fsck from a rescuecd to work again if that
makes a difference)

Any ideas on where the missing gb are?

Thanks