3.6.5pre has been in dev and testing and not all of the changes put in it were validated so it isn't ready to ship yet. Rather than pushing out code that wasn't fully tested, or releasing a version called 3.6.5 that included different code from 3.6.5pre, they branched around it and released 3.6.6 with just the one change needed. I imagine they'll retire 3.6.5pre and move everything over to 3.6.7pre where they will continue working on the next release.