I am using a CMS-provided menu:

When I select most of the options, the menu does some nice "context highlighting" to show the user where they are in the "site map":

But when I select either element on the last menu (Archives), BOTH items are highlighted, which I do not believe is the desired behavior:

(notice the darker gray background on both menu items).
So, is this a bug because this is the last drop down menu, or because there are only two items, or is it a bug in something I have done in a stylesheet to mess with the menu?
Thanks for any insight.