So it started with Windows 3.1 (or 3.11 for Workgroups). There was of course Windows 1.0 (released in '85) and Windows 2.X (1990) prior to that, but 3.1 was what I used when I first started into computing.
From there, it goes as follows:
Win 95 = Version 4
Win 98 = 5
Win ME = 6
Win XP = 7
Vista = 8
Win 7 = 9
That doesn't even count NT/2000, which was also very much a Windows release.
The only explanation I can think of is that perhaps 95 through to ME were the same thing, but I certainly recall buying three separate sku's so that can't be it.
It's possible that they count all versions with a MS-DOS backbone as the same (Windows 1 through 3.11), but those were the last ones they bothered numbering. If they were going back to the numbering system, why disregard two of these three?
I suppose this is but a low-grade misnomer, but these are the people I'm trusting with my hardware - is it too much to expect them to count properly? Have I taken crazy pills? It's not that big a deal, but Microsoft has so much money, it's fun to prod them a bit. Stick it to the man!