Yep. Let's not forget:

NT 3.5 = kernel version 3.5

NT 4 = kernel version 4

Windows 2000= kernel version 5

Windows XP = Kernel version 5.1 AND kernel version 5.2

Windows Vista = Kernel version 6

Windows 7 = Kernel version 6.1 (WTF OMG so retarded)

Windows 8 = Kernel version 6.2

I know they want to be all pure about their major numbers and whatever, but really they should just get over it. Windows7 is a major release different to Windows Vista, regardless of whether the NT people think it's a pretty minor change.

There's also an app-compat thing going on as well: http://blogs.msdn.com/b/oldnewthing/archive/2004/02/13/72476.aspx but frankly in this case I don't think that's a good enough excuse.