For people knowing Lua a bit longer, the "divergence" is nothing surprising, it's an explicit and
very long standing design choice by Lua authors. Specifically, that's what allowed them to evolve Lua to the awesome language it is, by making breaking changes when they invented/converged on a notably better way to do something. Personally, I find it more appropriate to look at each new Lua version as a
wholly new language, rather than a "new release of the same language". A new language, which "accidentally", as a bow to the user, is very, very similar to the "previous language" from the Lua family (to the point of usually being API and syntax compatible).
Actually, the changes between 5.1, 5.2, 5.3 are super small and smooth compared to what was done between e.g. 4.x and 5.x, where language syntax was changed. Between 5.x versions, with some effort you can make your code portable, if you wish so. (I'd say that's what one would expect when seeing the major digit unchanged in a version string.) There are many libraries which take care to. I'm not 100% sure about this one (i.e. haven't checked), but from the top of my head, based on the author's reputation, I'd blindly say https://github.com/stevedonovan/Penlight should be a nice example of a library portable between 5.x versions.
The proliferation of the 5.1 version is sometimes semi-jokingly seen as a "curse of success" - that it became the first version to really nail it so well, that it spread so wildly, and the newer versions bring small enough changes, that some people find it hard to justify any effort at all to upgrade. But it's also perfectly OK: Lua 5.1 is a great language already, and can simply be seen as a done/completed work.