So I'm trying out this 'df' command in Linux and the numbers don't quite add up although I'm sure there's a reasonable explanation for this. So the 'df' command reports filesystem disk space usage.

Here's an example of the command that I try:

df -h /tmp

And here is the result:

Filesystem Size Used Avail Use% Mounted on
/dev/sda7 9.7G 33M 9.1G 1% /tmp

So I would have thought that Used + Avail = Size but that does not apply above as it just doesn't add up.

I'm just hoping some one may be able to explain why the result does not add up? What am I missing here?

I've tried this with other directories as well, not just /tmp, and the numbers are inconsistent for the other ones I've tried as well.