CentOS and RHEL will only be exactly the same at the moment when RHEL is a point-in-time fork of CentOS. As soon ad RHEL forks from CentOS, CentOS will roll forward and will no longer be exactly the same as RHEL.

Previously, CentOS was a rebuild of RHEL. In between RHEL releases, CentOS was exactly the same as RHEL. When RHEL had a release/fix/backpoint, CentOS trailed until it was rebuilt from the new RHEL source.

The "old" CentOS was exactly the same nearly always (nearly perfect overlap) and the "new" CentOS is exactly the same nearly never (almost no overlap).