By default, when you format a filesystem with EXT3, 5% of the drive is reserved for root. df accounts for this reserve when it reports what is available, while du shows what is actually in use.
You can view the reserved blocks by running:
tune2fs -l /dev/sda|grep -i reserve
and you will get something like:
Reserved block count: 412825
Reserved GDT blocks: 1022
Reserved blocks uid: 0 (user root)
Reserved blocks gid: 0 (group root)
If you would like to adjust that to a lower percent, you can do so with something like
tune2fs -m 1 /dev/sda
You can reduce it to 0, however since this is your root filesystem I would be wary of doing that. If the filesystem actually filled it may make maintenance tasks required to clean it up difficult.