I understand what you mean here, however we should look at the task the the layer (node) is responsible for.
The AO_MF_Pistol_Idle_ADS is a Aim Offset and to understand what it does you need to understand what additive animation does.
Additive animations are animations that take into account the current animation given to it (the input pose, in your case the output from the layered blend per bone node) and finds the difference between the rotation from the additive animation (In your case the Aim Offset) then adds that difference to the input pose so that it blends the two animations. Additive animations do not completely override a input pose but change it a bit. Its a very hard to explain topic haha took me a few months to understand however I’ll link you a video so you can have visual representation of this.
Aim Offsets
PS* aim offset is not a single animation rather a type of blendspace that outputs a single animation based on the input given to it, it chooses and blends from the animation inside it.