So, obviously Bitcoin Core 0.13.0 and 0.13.1 have been released, 0.13.2 is nearing release, and 0.14.0 will have its feature freeze in a few weeks. However, there are also some hundred odd nodes that show up as 0.13.99. What does that version represent?
While releases are carefully tested, tagged and then made available as executables, it is also possible to compile Bitcoin directly from the source code.
Bitcoin Core nodes that were built directly from a commit on the master branch instead of one of the releases show up as 0.XX.99, where XX is the latest major release.