2.1.8 is the version of the project, that any maintainer is free to choose as he pleases.
2.3.8 is the version of the library-interface. It tells you about API/ABI stability because follows the strict semantic versioning rules originally implemented by libtool. See the comment here:
And libtool docs here:
Sometimes we have bugfix releases that add new public functions. In this case, we must increment the minor version of the library-interface. Whereas for the "project"-version we would just increment the micro level (as those are just bug-fixes). In any case, those are two completely different versions, that coincidentally look very similar at the moment :)
Tom