How Does Detrimential Effect Duration Decrease Stack?

[San0ix]

I was playing around with my Skeleton Demonologist and realized that one of their demon seeds gives them a lot of detrimental effect duration decrease. Obviously I immediately thought that this, in combination with Resilient Bones would be awesome and I would get more then 80% (40% from the seed and 44% from Resilient Bomes) reduced durations.
Thing is, when I was running around I once hit by Arcidic Blood, which is supposed to last 5 turns. So in theory, that debuff should have only lasted 1 turn, pretty basic math. However, the debuff lasted 2 turns.

Does this mean that this effect has a cap or does it stack with diminishing returns? Or was it just a rounding error by the game?

[Micbran]

It's probably multiplicative, not additive.

[San0ix]

It's probably multiplicative, not additive.

That would work out if it multiplies the effect duration (in this case 0.6*0.56*5=1.68~2), thanks