It is just good coding on X-carts part (in the main). THere are no switchable styles or anything like that. The original style sheet doesn't use many things that are unsupported in NS 4.X.
As for user stats check
http://www.google.com/press/zeitgeist.html for global access data to google. Although there is no hard and fast rules, you can see IE 6 is the big one, but of course, just because it is only 3 % that use ns, thats still a lot of people if you are getting 1000+ hits a day.