[CentOS] Disk usage incorrectly reported by du

Wed Mar 19 12:14:04 UTC 2014
Radu Radutiu <rradutiu at gmail.com>

I have an ext4  filesystem for which the reported disk usage is not
correct.  I have noticed the discrepancy after I rsync-ed the content to
another filesystem and noticed that the used space on the target is almost
double of the size reported on the source.
Both machines are running the same software - with the same kernel version
and same coreutils version (which I later upgraded to latest available
version).
Both filesystems are clean (verified with fsck.ext4).
No sparse files.
After further investigation I think that the problem is most likely on the
source machine.
Here is the du output for for one directory exhibiting the problem:

#du -h |grep \/51
201M    ./51/msg/8
567M    ./51/msg/9
237M    ./51/msg/6
279M    ./51/msg/0
174M    ./51/msg/10
273M    ./51/msg/2
341M    ./51/msg/7
408M    ./51/msg/4
222M    ./51/msg/11
174M    ./51/msg/5
238M    ./51/msg/1
271M    ./51/msg/3
3.3G    ./51/msg
3.3G    ./51

after changing the directory and running du again I get different numbers

#cd 51
du -h
306M    ./msg/8
676M    ./msg/9
351M    ./msg/6
338M    ./msg/0
347M    ./msg/10
394M    ./msg/2
480M    ./msg/7
544M    ./msg/4
407M    ./msg/11
312M    ./msg/5
326M    ./msg/1
377M    ./msg/3
4.8G    ./msg
4.8G    .

Do you have any idea what could cause this behaviour?