I think it's just marketing. Vista was NT 6, so
that's one possible reason. But 7 is NT 6.1.

I'm sure they'd call it Windows Garbage Truck if
they thought it would sell more copies.
1. Win3.x
[ NT 3.5 and 3.52 ]
2. Win95 Platform 1, v. 4
NT4 Platform 2, v. 4
3. Win98 Platform 1, v. 4.1
2000 Platform 2, v. 5
4. Win-ME Platform 1, v. 5
5. Win-XP Platform 2, v. 5.1 [NT]
6. Vista Platform 2, v. 6 [NT]
7. Win7 Platform 2, v. 6.1 [NT]
It's actually 2 different platforms, officially --
Windows and Windows NT. Windows ended with
ME.
I hear "Windows 8" is NT v. 6.2, which may
have more to do with backward compatibility
than anything else. But there is a certain amount
of sense to the numbers: Win98 was an update
to Win95. Thus 4.1. WinXP was an update to 2000,
so it's 5.1. Win7 is an update to Vista, so it's 6.1.
WinME is a bit of an anomally. It was a very minor
update to Win98, but it did have some core changes,
too. I don't know whether calling it v. 5 was actually
justified or not.