Server With RHEL4 Or Centos 4.4

i have a problem with all my server with RHEL4 or Centos 4.4, when u upload small files on the server the system report a different size, this is a little example

> drwxr-xr-x 2 user user 4096 May 10 11:11 ./
> drwxr-x--- 12 user nobody 4096 May 10 11:16 ../
> -rw-r--r-- 1 user user 7514 May 10 11:11 cred.js

total is about 7+4+4= 15kb

but when i do du -h in the folder

root@ares [/home/user/www/]# du -h
20K

where are this 5kb of difference? i have note that this happend when a site have small files inside, example when u install joomla about 7mb on my server it is 18Mb, the server size block is set to 4kb