You do have a point, but I don't know that I am wrong, I was fairly sure that I was right, to a degree.
The site you linked to had things like Local Area Security Linux, Damn Small Linux, various rescue cds et all which are hardly what one would consider to be desktop linux distros, they are more specialised niche distros. The list was too comprehensive.
IMHO when someone asks "which desktop linux distro" the answer (atm) is either Ubuntu, Debian, Fedora, OpenSUSE or Mint. Those are the top 5 (in no particular order).
Windows 3.1 (not sure what was before that apart from dos :) )
Windows 2000 } think these are the same by different names but meh.
Windows ME }
Windows XP Pro
WIndows XP Home
Windows XP Media Center Edition
Windows Vista Home Basic
WIndows Vista Home Premium
Windows Vista Business
Windows Vista Ultimate
Have I missed any? Thats more than 5. Granted not many will be using 3.1 or 95 etc but even if you disount anything before xp, its still more.
The reason so many linux distros exist is because its free to modify, and people who need it to do very specific things can mod the distro and create their own relatively easily, and they make those changes available back to the community. If you could do that with windows I'm sure there would be more (and better) versions of it around.
The point here is that if you get a list of all linux *desktop* distros, I'm sure there would be more but there are plenty MS distros too. I don't see it as an issue, the reason its an issue for wintards is because they are force fed a single current OS when they buy a new pc and thats the end of it, they dont realise there is a choice.
no trolling intended, honest.......