I hope you might find that the Steam hardware survey isn't monitoring a majority of the computers out there. These are also "old" numbers already, you'll typically find the numbers lagging a month and this is data from february ... and DiRT2 only coming with Cypress and Juniper helps the numbers.
Yes yes of course, steam is just a subset of the total users and not all users are online at a given point of time. But statistically the numbers are valid, its just like doing a survey in a city or town. What does Dirt2 being bundled with Cypress and Juniper have to do with anything though?
And Juniper has been on sale since october, just three weeks after cypress. I dunno about you but i was very surprised to see that cypress had outsold it.
Of course this happpened with the previous gen too with 4800 series far outselling the 4600 series(in steam user stats anyway) but the 4800 series sold for less than 200 USD for a majority of their life