This diagram is an illustration of how Open Source development keeps moving. Visualize the (mostly) blue diamond as moving through time (here, from left to right). The "release" points are static: largely unchanging snapshots of the evolving code base. What was the "current" stable release of a project will age with time, its development activity displaced by work and attention on newer releases.

[Note: Where the "Sweet Spot" resides, and how broad it is, depends upon the open source project, pace of development, community practices, etc. In Drupal, once a release gets more than two stable releases behind what is the latest and greatest, there is significantly less community development work and support for that release. The specifics for other projects would be different.]