Thunderlord's Decree Damage

I was playing with Thunderlord's Decree and after looking at the damage it does (by hovering over it on the screen when you hit TAB) and it showed that it was only doing 10% of my AP and 10% of my bonus AD. I'm not sure if this is a display bug, or if it was actually dealing less damage. I also don't know if it is supposed to be 20% of my AP or my bonus AD because it is different in the LOL client and on the wiki.