The canonical explanation is:
Windows 1.0 -> 1
Windows 2.0 -> 2
Windows 3.x -> 3
Windows 95/98/Me -> 4
Windows 2000/XP -> 5
Windows Vista -> 6
Windows 7
I was at Bic Camera (huge electronics store) recently, they were running a promotional video (that looked like it had been made by MS Japan), and it listed the "numbers" off like that.
Which does mean that, until Windows 7 itself, the "number" matches the major version number.
Larry Osterman told me he expects Windows 8 to be version 6.2 