Inconsistent highlighting on dropdown menu

I have a valid XHTML 1.1 document with valid CSS controlling the styling.
http://www.danrumney.co.uk/menuDemo.html

As you will see, it's a drop down navigation menu.

I'm viewing it in Firefox v1.5 on Windows XP... IE users will not be
able to help due to the fact that IE6 doesn't support the :hover
pseudo-class on anything but anchors.

If you roll over "web technologies" and then over the sub-menu items,
you'll see that *only* the text is highlighted. However, if you do the
same in the "personal" menu, the whole line is highlighted. Also, in
this menu, if you roll off the text of the sub-menu item, the whitespace
is highlighted, but not the test (try the page... it's clearer that way)

Can anyone offer any suggestions as to why the behaviour is inconsistent
in this way?

Thanks

Dan