RH9: discrepancy between free disk space info: df vs. du, why?

[Rade Trimceski]

Hello all!

I'm running RH9 on a Sony Vaio laptop. I use ext3 for file system and
2.6.7 kernel. I'm having this puzzling problem with my free disk space. 

I run java based simulations which can generate pretty big files (900MB
~ 1.2GB). Lets say that I reboot the machine, using df -h I see that I
have 1.4GB free disk space, and I leave it running for 4 days. In the
course of 4 days I create and delete about 40 to 70 files. After
deleting all the simulation related files (after 4 days of uptime) df-h
reports only about 123MB free disk space. However if I run du on / I can
deduce that the free space is indeed 1.4GB. If I reboot the machine df
-h reports 1.4GB.

My qeustion to you: what causes the discrepancy between what df and du
report and how can I fix this without reboot? If I look at the df output
where does my free disk space go when I delete all the files?

I've tried to run updatedb to no avail of fixing this. I've also checked
if /var or /tmp and other dirs get bigger and no directory seems to
account for the "lost disk space". I'm suspecting this has something to
do with the journaling capability of ext3. Has anyone experienced
anything similar to this?

If people don't have an answer but they have an idea on which list I can
get help please let me know :).

Thanks 
Rade