2000 (5), XP (5.1), Vista (6), 7 (6.1), 8 (6.2), 8.1 (6.3)
And Windows 10 will be 6.4.
It just doesn't make any sense.
One popular theory is that there is software out there that checks for Windows version == 6 instead of >= 6 when deciding to enable features for Vista and above. This was definitely true of libcurl at one point, for example.
In that case, my vote is for Windows X to be version 6.3.1.