>The dates are likely based on when the ISO was actually created. Therefore, if the ISO was generated on Oct. 1st and no issues were found with it in QA, then the date you are seeing on the mirrors is correct. The ISOs are based on the original 5.4 tree and don't include updates that Red Hat released after the initial release of RHEL 5.4.
As far as I remember, some subsequent RH updates are rolled-into the CentOS release. Someone more familiar with the build process may clarify that.