Wether this is a problem or not, Centos only supports the most current release of each version. With Centos 5 (not 5.0) the only supported version is 5.5. When each new release (5.4 to 5.5) comes out the system is normally updated to the current relase level automatically. If you run:
and it does not come back 5.5 then someone has altered the setup. This means that you would not have gotten any of the updates since 5.3 was released.