Version 1.3 was an attempt to continue the development of PunBB 1.3 Beta. Right now it is still under development, but that will only continue to a point where all known bugs are fixed, so that people who want to use it may have a pretty stable version.
The reason for this is that the FluxBB Development Team had not been too happy about some things in this version, for example the way the extension system works.
Please note that 1.2 mods and styles will not work on 1.3.