A long-standing oddity of Windows is that its branded number has for some years now not matched the version number stamped into the kernel and other parts of the operating system. Windows 7, for example, reported itself to software as being version 6.1. Windows 8 is 6.2, and Windows 8.1 is 6.3.
Current public builds of Windows 10 repeat this trend – they purport to be version 6.4 – but not for much longer. Chinese site ITHome published a picture showing the version number to be 10.0. Version number 10.0 is also cropping up on BuildFeed which tracks build numbers, and has been further corroborated elsewhere.
Interesting little tidbit of information.