I'd be surprised if more than 50% of computer users (55% IE * 90% Windows) changed their default browser. The vast majority of computer users are non-technical people, and they use the default browser unless someone changes it for them.
But they also browse (far) less compared to people who use alternate browsers. So statistically, your sites have a much higher chance of registering Chrome and Firefox users than IE users.
Add: OTOH, I think StatsCounter, Clicky and Wikimedia stats are more useful. They represent the "active" Internet population better. The ones who are more likely to come to your/any website.
It's also so important to filter down to geographic regions you care most about when looking at these stats. It's common to see developers/companies making browser support decisions based on global stats, even when they often derive most of their profits from a much smaller region (with different browser usage stats).
But they also browse (far) less compared to people who use alternate browsers. So statistically, your sites have a much higher chance of registering Chrome and Firefox users than IE users.
Add: OTOH, I think StatsCounter, Clicky and Wikimedia stats are more useful. They represent the "active" Internet population better. The ones who are more likely to come to your/any website.