[gameplay] Muramana not giving correct bonus ad

I'm not sure if it's champion specific or a general issue with the item itself, but the few games i bought manamune/muramana on blitzcrank, it would say for its unique passive, awe "gives 2% of max mana as bonus ad", but it only gave 1% - I had a total of 3847 mana, but it only gave me 38 bonus ad (instead of 76).

I tried to take a screenshot in-game, but once the game was finished the screenshot just came out black.