Why 'df' is reporting invalid Disk Usage Summary?

Issue

• df command shows less space than the actual partition it has,

        /dev/mapper/VolGroup00-LogVol03
         69G 180M 66G 1% /mnt

        # lvdisplay /dev/mapper/VolGroup00-LogVol03
         --- Logical volume ---
         LV Name; /dev/VolGroup00/LogVol03
         VG Name VolGroup00
         LV UUID Uo8Bb7-fEol-3RlL-Q3JV-a0Rv-CeTH-PjLEHj
         LV Write Access read/write
         LV Status available
        # open 1
         LV Size 70.00 GB