Simple answer:
Microsoft brands Windows releases by a name ("Windows Server 2008 R2"). The internal version number (6.1) and build number (7601) have not been used in marketing since Windows NT 4.
Canonical brands Ubuntu releases by both a name ("Lucid Lynx") and the date of release (10.04).
Apple brands Mac OS X releases by both a name ("OS X Lion") and a version number (10.7.2).
Just as the Ubuntu names are either synonyms of their version numbers (ubuntu-hardy to ubuntu-8.04) or non-existent (ubuntu-lucid vs. ubuntu-10.04), so should the Mac OS X names either be synonymed or blackholed.