Quoting OliverFA_306, reply 5
Interesting. I thougt that bonus would be multiplicativa and not additive
I was under the impression that everything percentage in this game was additive
Many things are, many aren't.
For instance, the Accuracy and Dodge bonuses from the Lucky faction trait are multiplicative, as is the Dodge bonus from the Shrink spell, but a lot are additive when you wouldn't expect.
For instance, I just noted that the Vital Strike series of bonuses are additive (on crit chance, at least; there is no visible stat for crit damage). I'd fully expected them to be cumulative, like with Reap!