You are right of course, it’s not intuitive. To the point that one wouldn’t even suspect that these two even might be different, to begin with.
Still, as a general guide, I can recommend this excellent article from Iria: What is Additive/Multiplicative Damage; A Modern DH’s Perspective. Although it primarily focuses on DH, it contains a lot of class-agnostic information and explanations of internal mechanics. Worth making yourself a bookmark, to be sure.