What you measured is just the overlap between minor releases of the same major release. It helps to think of them as service packs if you want a MicroSoft analogy. So each minor release is supported until it has be surplanted for 3 months by a new one on the same major release line or the whole major release line goes end of life.

Sure, but the point is that each minor release contains changes in all third party open source packages/ports by taking them to the head version.

Open source packages often include breaking changes, all but guaranteeing your application to fail. With (a paid version of) RedHat Linux, RedHat modifies the open source packages to remediate CVEs by modifying the original version.