I believe it is because Apple stopped branding their operating system as 'Mac OS X' in favour of just 'OS X'. And I'm now assuming they're attempting to phase out version numbering before they hit either OS 11 or 10.10.
The official MacRumors reason for this is that Apple isn't using the number in the branding of OS X, nor are they using 'Mac' as a part of it either. This is why 'Mac' isn't part of the forum name and why '10.9' is in parentheses.