Disk space usage doesn't add up with df & du
Solution 1:
You probably have some deleted big log file, database file or something similar lying around, waiting for the process holding the file releasing it.
In Linux, a file deletion simply unlinks the file. It actually gets deleted when there's no file handles connected to that file anymore. So, if you have a 2 GB log file which you delete manually with rm, the disk space will not be freed until you restart syslog daemon (or send HUP signal to it).
Try
lsof -n | grep -i deleted
and see if you have any deleted zombie files still floating around.