Since the times of version 10.2, the internal code name of OS X major releases has become public knowledge and Apple has started using it in marketing. While there is a whole series of arguments for and against the usage of a non-sequential version numbering, I would say that in the case of operating systems it works just fine. After all, people only have to remember what the current release’s name is, and maybe the names of the two that came before it. Not a big deal.