df says disk is full, but it is not

On a virtualized server running Ubuntu 10.04, df reports the following:

# df -hFilesystem            Size  Used Avail Use% Mounted on/dev/sda1             7.4G  7.0G     0 100% /none                  498M  160K  498M   1% /devnone                  500M     0  500M   0% /dev/shmnone                  500M   92K  500M   1% /var/runnone                  500M     0  500M   0% /var/locknone                  500M     0  500M   0% /lib/init/rw/dev/sda3             917G  305G  566G  36% /home

This is puzzling me for two reasons: 1.) df says that /dev/sda1, mounted at /, has a 7.4 gigabyte capacity, of which only 7.0 gigabytes are in use, yet it reports / being 100 percent full; and 2.) I can create files on / so it clearly does have space left.

Possibly relevant is that the directory /www is a symbolic link to /home/www, which is on a different partition (/dev/sda3, mounted at /home).

Can anyone offer suggestions on what might be going on here? The server appears to be working without issue, but I want to make sure there's not a problem with the partition table, file systems or something else which might result in implosion (or explosion) later.