The PGDG RPMs for RHEL 8 are typically built against the most recent minor release available (currently 8.10). Because Red Hat Enterprise Linux maintains ABI compatibility across RHEL 8 minor versions, packages built on 8.10 remain compatible with earlier minor releases such as 8.9, 8.8, etc.
For that reason, the package naming may include the ".rhel8.10" suffix even when the RPM is located in repositories corresponding to earlier minor versions like "rhel-8.9-x86_64".
In short, the suffix reflects the build environment rather than a strict requirement for that exact minor release.