Hello,
Major version numbers are incremented every three months. This has been the case since the FP 12 release earlier this year.
Flash Player 13 is what we call the Extended Support Release (ESR). The difference between the ESR and the current version is that the ESR is only updated with security fixes and the latest version has security fixes, general bug fixes, and new features. The ESR is mainly geared towards enterprise customers who don't care for new features, but want security updates.
HTH.
--
Maria