yes weird one.
Here's a guess.
The widths are causing a problem. 80% plus 40px may be more than the containing width.
What I would do next ( as I say I am on my laptop so don't have full access to IE) is add the following to the bottom of the stylesheet and see what it does:
Code:
#categoriesContent{border:1px solid red;position:relative;}
a.category-top{border:1px solid purple;width:130px;}
It adds some borders in so we can see more easily, reduces the width of the <a> and makes the containing element position relative just to be on the safe side.
Leave it up like this if it doesn't have a good effect so I can see the results in IE.