[CentOS] 196 packages to install or update ??

Hi, today I did a yum upgrade and see that there was 193 packages to 
update and 3 packages to install...
I always upgraded my server (with yum) on a weekly basis so I don't 
undestand how it is possible to have a so big update.

Is there something wrong with my yum setup ? A mix between different 
versions of CentOS ?

I'm running CentOS 4 i386

Thank you