Specific question about "/mnt": /mnt is not showing the disk space

Question

The disk space showing from "df -h" and "du -sch ./*" seems different for "/mnt". Can somebody help me in finding the probable reason?

===============================

    #df -h
    Filesystem                                      Size  Used Avail Use% Mounted on
    /dev/mapper/rootvg-rootlv                       19G   15G  2.7G  85% /
    /dev/mapper/appsvg-appslv                       60G   23G   34G  40% /apps
    /dev/sda1                                       290M   32M  244M  12% /boot
    tmpfs                                          3.8G     0  3.8G   0% /dev/shm
    //165.2.141.31/IHGJavelinTest                 12G   11G  1.4G  89% /mnt/mdmfile
    adcsan5:/vol/etl_dev                            40G  7.0M   40G   1% /apps/mdmfile
    adcsan5:/vol/ess_dev                            40G  7.4M   40G   1% /apps/fileshare

 =============================
 #cat /etc/redhat-release
  Red Hat Enterprise Linux Server release 5.9 (Tikanga)
 ================================================
    #du -sch ./*
    0       ./a
    23G     ./apps
    7.8M    ./bin
    22M     ./boot
    108K    ./dev
    66M     ./etc
    1.2M    ./home
    351M    ./lib
    27M     ./lib64
    0       ./local
    16K     ./lost+found
    8.0K    ./media
    **216K    ./mnt
    5.2G    ./opt
    0       ./proc
    642M    ./root
    39M     ./sbin
    8.0K    ./selinux
    1.2M    ./srv
    0       ./swapfile1
    1.1G    ./swapfile2
    8.5M    ./swapfile3
    0       ./sys
    3.6M    ./tmp
    4.0K    ./u00
    3.1G    ./usr
    3.1G    ./var
    36G     total
==========================================    

    #du -sch /mnt/*
    4.0K    /mnt/mb-arch
    4.0K    /mnt/mdm
    200K    /mnt/mdmfile
    208K    total
============================================

    #du -sch /mnt/mdmfile/*
    60K     /mnt/mdmfile/archives
    12K     /mnt/mdmfile/IHGJavelinDev
    4.0K    /mnt/mdmfile/IHGJavelinTest
    84K     /mnt/mdmfile/statistics
    160K    total

Thanks in advance.

EDIT:: My question is why /mnt showing 12Gb when checked using "disk free"(df) command and I was not able to see it when using "disk usage"(du) command. This is the case for "/mnt" only, so I think these is some explanation related to mounting concept.