Same partition but different partition size were reported by hardware browser and df

[Barry Yu]

One machine I installed fc3 in a partition where 25 gb was reserved, I 
found that;

Using hardware browser to see that partition, size 25gb was reported ( 
This is correct size)

Using df  -h   /dev/hda10;

    Filesystem      size      used      Available      Used %      
Mounted on
    /dev/hda10      15g      6.9g      6.9g               51%            /


Using fdisk;
                               Start         End               Blocks   
               Id
    /dev/hda10           4264         7451            25607578+         
83      linux

What is wrong ? This machine after used for a week, a message telling 
that the partition is full (15gb is used up), and I found that under 
/mnt/ms/ folder there were still some contents supposed should have been 
umounted, even I umount it again, the contents are still there! I think 
it has something to do with misreported partition size as above mentioned.