Thunderlord's Decree's tooltip when highlighted over shows only 10% of bonus AD

I play Katarina a lot. With Katarina, I use Thunderlord's Decree. I have been experimenting with a Hextech Gunblade build, so I check Thunderlord's Decree in my scoreboard frequently. Currently, Thunderlord's Decree says that it gives 20% of Bonus AD in bonus damage added. However, when highlighted over, it appears to only give 10% of bonus AD in magic damage.

First, I checked my AD, with a total of 65 Bonus AD. I highlighted over Thunderlord's Decree in the scoreboard and saw 6.55 damage from Bonus AD. That's only 10%, where it should have said 13 from Bonus AD. I don't know if this is only a tooltip bug. It could be, but it could also be slightly game changing bug.

I don't have screenshot evidence because I'm not in game, but I'll try to edit it in shortly never