The way to think about OS X versions is that the "10" is a marketing prefix, or an 'epoch' number - "10.5.8" should be read as "Mac OS X version 5, point releas

The way to think about OS X versions is that the "10" is a marketing prefix, or an 'epoch' number - "10.5.8" should be read as "Mac OS X version 5, point release 8". It seems that Apple is trying to transition away from "version X (with cute codename Y)" to "Y (with implementation-detail version X)", much like Microsoft has been doing with Office, and Canonical has done with Ubuntu.