Avatar of the Slaughter + Disciple of the Slaughter

I'm playing a raider with both Avatar of the Slaughter and Disciple of the Slaughter Passives. Each gives 8% increased attack damage per frenzy charge but in my character panel under charges, it tells me I only get +8% attack damage modifier per frenzy charge (I'd expect +16%).

Is this a bug? Or are the two not supposed to stack? Or am I missing something here?

Thanks!

Disciple of Slaughter doesn't have "attack" wording, meaning it applies to spells too. So I'm guessing perhaps there should be separate sentence listed in the panel.