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.

WRT version names & numbers:

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, and our tags use the names.

Canonical brands Ubuntu releases by both a name ("Lucid Lynx") and the date of release (10.04). The release date seems to be more prominent, and our tags use the release numbers.

Apple brands Mac OS X releases by both a name ("OS X Lion") and a version number (10.7.2). The name is more prominent.