Well, the true is the menu is not made well for PC. Is not pain in the ass and for a couple of hours you shall forget easy for this. Of course you can always use some mods to fix it: )
This is gonna be tricky to explain.
Sometimes it does that. It refuses to stop highlighting something. What causes this sometimes and not others, I cannot say. But there are two solutions. First, is to use the directional buttons. This always works.
The second is to use your mousewheel. Ive had the most success hovering teh cursor over the selected topic, and then moving the mouse wheel up and down. This usually frees it up, and lets you navigate normally with the mouse. Other times it doesnt allow free movement, but will let you scroll through, highlighting each topic as it moves. And sometimes it simply does not work at all.
There's no fix for it, short of using mods.
Mouse input for menus is slightly broken in Skyrim, http://www.nexusmods.com/skyrim/mods/27371 partially fixes the issue with dialogue menus but the best thing to do is get used to using keyboard input for menus