Where is the missing disk space gone?

[Michael Yang <michael.yxf@gmail.com>]

Lenny/Sid, kernel 2.6-22, I had my system re-installed twice, due to 
some problem.

The /home partition was always kept unformatted and mounted during each 
installation. Now I found the free disk space is not correctly shown. I 
don't know if it's a display mistake or if there are any free space 
physically missing. I used "df -h", it returns to me with the follows:
/dev/sda8              19G  334M   18G   2% /home

When I used "du -sh /home", it returns to me with the actual used spaces:
162M    /home

There are almost half of the spaces in dark. How could I diagnose this 
and get the disk space back to normal?

Your helps are appreciated!

Thanks.
Michael