I think 98 is considered a point release for 95 (so if 95 was 4.0 then 98 would be 4.5), 2000 was part of the NT chain so take that out. And then it actually works... well it does if you forget ME, which MS seems to have written out of history.
95 = 4.0
98 = 4.1
ME = 4.9
2000 = 5.0
XP = 5.1
XP 64-bit/Server 2003 = 5.2
Vista/Server 2008 = 6.0
7/2008 R2 = 6.1