Traditionally Drupal was developed in two parallel branches. Take the example of Drupal 7 and 8. There was the supported stable branch of Drupal 7 that received bugfixes only and there was a development branch of Drupal 8 where developers went free to change and improve things the way they have seen best. As the development of Drupal 8 was nearing release, contributed module and theme authors were asked to take a look and release new versions as well in preparation. While the new branch was built with great care, it was not proven yet in various production environments. It also often took quite some time for contributed modules and themes to update.

Traditional Drupal code vs. module compatibility