It really is ridiculous how these numbers are bandied about by people as being representative of the real world.
It's nearly impossible to buy a new PC that doesn't come with Windows, unless you build it yourself.
I am willing to bet there's a noticable percentage of Wintel boxes sold that are immediately turned into Lintel boxes by people who know enough about software to install some Linux flavour (not that you need to know a lot to stick in a Linux LiveCD and wipe your drive), but not enough about hardware to build their own PC from scratch. And even if you're talking specifically laptops, not desktop PCs, once again it is practically impossible to buy a big-name laptop without Windows on it, so there's another "sale" for the PR campaigns.