http://www.extremetech.com/computing/191279-why-is-it-called-windows-10-not-windows-9
There’s also the fact that the name of each Windows release doesn’t actually match the real version number. For example, Windows 8.1 is actually version 6.3 of Windows. Windows 10 is version 6.4. The last time the release name actually matched the version number was the enterprise-focused Windows NT 4.0, which was released back in 1996. Windows 2000, which was called NT 5.0 during development, was actually version 5.0. Windows XP was version 5.1. Windows Vista was 6.0, Windows 7 was 6.1, Windows 8 was 6.2, and Windows 8.1 is version 6.3....
I remember when WinWord went from version 2 straight to version 6!