There are multiple reasons why it would work in IE9/10. And why they would have the IE prefixes.

a) They may be using compass (compass-style.org). With compass you get mixins which create rounded corners and use browser prefixes. So compass knows how to just make it work.

b) IE9/10 are pretty decent at rendering, so you get the benefit of a decent browser that probably just works. But if there's a bug, they don't give a shit.

c) It's expensive getting hardware to test on, taking time to test it, making sure you have a windows installation, they may just have macs and test on Chrome/Safari/Firefox/iPad. Chrome makes the claim that if it renders on Safari, it probably renders on chrome and vice versa. So lots of freebies.

d) DEBUGGING: To debug in any browser I can do it on a mac (decent dev env) to debug in IE I need a dev environment in windows. Enjoy setting that one up.