More than one wiz - Flame blades mirror image build

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.