The simple explanation is that Semantic Versioning is not at all the only way to do versioning, it is just one standard amongst others. Of course semver is progress in the sense that it has a specification that means you can assign meaning to the versions, but hey, so does the proposed versioning scheme for GTK. It is just not semver.

I think it is interesting to think about what the versioning scheme says about the development model and methodology of the project:

Linux currently uses even-odd versioning, but regardless tries to never break APIs or ABIs https://en.wikipedia.org/wiki/Linux_kernel#Version_numbering After much debate, Linux switched from a model where the major number never changed, which is kinda inline with semver. But since this seemed increasingly silly, to have a 3 there forever, they decided to just throw this overboard and start incrementing the version number more often. (This discussion was much more interesting in reality, read LKML for details).

Android uses an interesting brand-version and actually-useful-version scheme (API level) https://en.wikipedia.org/wiki/Android_version_history This very much reflects how versioning and releasing a new version has become a PR event. It is really funny how this distinction is even reflected in code.

And then of course there are the funky oddballs: https://en.wikipedia.org/wiki/TeX This asymptotically more precise number is suitable for a project that has the idea that it is approaching perfection without changing much. Not sure if every TeX user out there would agree, but it indeed says something about the author =).

There are plenty others, and of course it can be confusing if you think that semver is the only game in town. Perhaps the ideas expressed in the semver spec simple does not suit everyone.