If we’re going to include 2000 because XP brought in the NT kernel then we should really drop 95, 98 and ME because they’re from the non-NT chain. So it would really be NT4, 2000 (5), XP as 5.x, Vista as 6.
Really it’s MS numbering, they just make crap up.
The 95 line is in there because someone mentioned it. The MS numbering does make sense. After 3 there was 95 so it gets 4, and the rest were incremental updates worthy of a dot release. 2000 was a big change from NT, so got a whole number, while the incremental changes of XP and 2003 got dot releases. Vista was a big change from XP (good or bad), so got a new number, with 7 getting a dot.
I am among those who isn't happy that the original 7 project didn't come through. I was expecting that complete rewrite with the MinWin kernel, but just got a dot release over Vista.