They wasted no time, lol
This is normal for software development. It's not linear.
It doesn't go "develop - stop - release - develop - stop - release".
There are several "develop" branches that co-exist in parallel. Only once a branch reaches a certain stability and completeness does it gets released, but already several other branches have moved forward, and then those changes get folded in when they are ready. Quite often different teams are assigned to different branches. Some branches might be for new features, some might be bug fixes. Software development is not just "one thing", "one project", but many projects that are being coordinated together. That's why Apple was able to create a build of iOS 12 without Group FaceTime, because it's being developed in its own branch and can be included or excluded, as needed.