[GAMEPLAY] Thunderlord's Decree incorrect ratios.

Hey Riot, I was playing one of my favorite champions, Katarina mid (no hate please, I'm here to report a bug), and I picked up Luden's Echo as my first item, as I usually do when I'm ahead. I was running the new Thunderlord's Decree mastery because it's essentially a free luden's and it would add a lot of burst to my combo, and as a long time Kat player, I felt it was way better for Katarina's general MO than Deathfire Touch. In the tooltip and on official websites, Thunderlord's Decree is layed out as a I specified earlier (10 per level) + 20% Bonus AD + 10% AP. I finished my Luden's Echo up, and the damage on my Luden's Echo tooltip in game was displayed as 100 (+13) magic damage. Then I checked my Thunderlord's Decree tooltip and it was displayed as 100 (+26) (+0) magic damage. If Thunderlord's and Luden's have the same AP ratio, they should come out to the same damage, right? It's only logical. Now, since the magic damage stacks with all my abilities, it was hard to tell whether the actual damage that was being applied was being coded correctly. This is worrisome because 10 per level + 20% AP + 10% Bonus AD is a shitload of damage. 20% AP is a lot to add to the combo of an assassin or burst mage.

So, TL;DR, I am 100% certain that Thunderlord's Decree's tooltip is being displayed INCORRECTLY, however, I am not sure whether the actual damage is being coded correctly or not.