bug#11597: df calculus wrong?

[system]

Hello everybody, 

first of all let me say a big thank you -and all the others- for
spending time and life donation to the GPL, FSF, ....  world of Linux. 


I am not an developer but mathematics is one of my favs so how can the
following explained? 

Command issued 
df -h

response 
----paste start -----
Dateisystem            Größe Benut  Verf Ben% Eingehängt auf
/dev/sda1              36G   25G  9,1G  73% /
....
----paste end -----

Circumstances and conditions: 
Ubuntu 10.04 lts server
SDA is a physical SATA connected drive 
Installation language de
Execution is done via ssh due to headless server system

If size (Größe) is 36GB and occupied space (Benut) is 25GB -guess-
remaining space (Verf) is below 10GB there must be  a bug in calculation
or in the way the first two values were derived from the system. 
If there would be a diff of +-1 digit nobody would think about but in
this case there is a mismatch in the msd (most signifikant digit) 


I would really appreciate feedback that the mail is recieved and if
there will be actions taken upon. 

Sincerely 
Sterzenbach