With Redhat Linux Enterprise 6 System the Df and Du

With Redhat linux enterprise 6 system the df and du commands allow you to see the mnt capacity, as shown below.  What causes the capacity shown to not be the same? redhat: ~ # df -h / mnt Filesystem Size Used Avail Use% Mounted on / dev / sdb 1008M 514M 443M 54% / mnt redhat: ~ # du -hs / mnt 40k / mnt

A) The mnt directory mounted block device is abnormal
B) The mnt directory is also mounted another block device and some files that are currently open.
C) The mnt directory have been deleted
D) This is a bug of the known du and df commands that need to be upgraded to the latest version