Board Thread:Questions and Answers/@comment-43863986-20190918133245/@comment-27971189-20190919115913

Card performance buff are increasing anything related to said card. So there's no surprise if the NP Damage Up buff also increases damage dealt by 50% like ATK Up 50% do, the purpose is increase damage, after all. What's wrong with that? LOL.

Then, damage calculation in FGO worked like this:

If you have two buffs that say they increase ATK, add them together to see the total benefit.

If you have two buffs that say they increase different things (e.g. Atk Up and Buster performance), multiply them together to see the total benefit.

Example:

A character has a +20% Atk buff, a +50% Atk buff, +20% Buster Buff, and +30% Buster buff.

Its equivalent to +70% Atk buff and +50% Buster buff, resulting 1.7*1.5 = 2.55 final damage.

That's why your experiment resulted like that:

Assuming the increase by buffs are same, 150% by three different buffs > 150% by two different buffs > 150% by one kind of buffs.

At glance it looks bad but sounds-niche-buffs like Increase Damage Dealt and NP Damage Up easily found its place when you start buffs stacking as it will help you achieving higher damage through merit of multiplicationS; although you might argue casual players won't dig deep to this.