A kernel module doesn't have to match the kernel version, it has to be able to resolve all the symbols (function calls, variables etc) it uses into valid symbols supplied by the kernel you are loading on.

The greater the difference between the kernel version you compiled for, and the kernel version you are trying to load it on, the greater the chance something you are relying on changed and the module loader cant resolve all the symbols and so it fails.

So saying a kmod has to match the kernel version is good practice but the reality is not quite as strict.

Red Hat has a list of "white listed" symbols that they try to maintain across a major version of RHEL so if your kmod only relies on them and nothing else then it should load on any kernel version within that release. But that's a Red Hat thing, not a Linux kernel thing.