Love it or hate it, Internet Explore for Windows is the main browser on the web at present. This means that at some point you will have to deal with IE's incorrect interpretation of the box model.
In simple terms this means that some versions of IE render the box model to a different width than other Standards browsers. So, your CSS styled list may look narrower in IE compared to other browsers.
The diagram below shows that if a content box is set to 200px wide with 20px of padding and a 20px wide border, the correct method of calculating the overall box width is:
content (200px) + padding (20px+20px) + borders (20px+20px) = 280px.
However, some versions of IE calculate the width as:
content, padding and borders together = 200px.
The solutions are to work around the problem using nested divs or use one of many box model hacks.