Have a look at this in your modern browser, and then try it in your IE (I’m guessing you are using IE 7):
IE 6, 7 and even 8 barely get anywhere.
Now have a look at these CSS 2.1 browser selector and declaration compatibility results:
As you can see, IE 6 and 7 barely support any of these common CSS selectors.
This little rounded-corner annoyance is happening because IE 6 and 7 don’t render websites the way they should be doing so.