jQuery targeting an element I don't want targeted

jQuery targeting an element I don't want targeted

I need to pull the submenu under the last element "inside" the box but ONLY when it's using the widescreen menu. To do that I need to grab the last element in the navigation, the submenu under it and then calculate how much the submenu needs to be pulled to the left. That I have no problem doing.  My problem is I can't seem to isolate ONLY the navigation element in ONLY the #top-menu ul.

The first step was just to isolate the last child element in the navigation. I'm stymied at the first step which is to isolate JUST that one element.    

[code]
jQuery('#top-menu > li:last-child').addClass('targetThisHere');
jQuery('#top-menu li.targetThisHere').css('backgroundColor','purple'); // just to display

[/code]   

For the moment, I've colored it purple to make sure I've grabbed the correct element. On the surface, everything looks fine. The problem is that if you collapse the screen size and the smaller screen "hamburger" appears, the menu for the smaller screen becomes available.

If you click the menu open with the hamburger, you'll notice that the new class has been added to THIS last-cihild element even though it's NOT part of the #top-menu element. The id for this element is # mobile_menu. I can't figure out why the class is being added to this element. I definitely don't want it to be.

Anyone have any ideas?