Why does layered blend per Bone completely override base pose?

I’m trying to add two animations together. I was under the impression that layered blend per bone was the node to use for this. I have an blendspace that raises an arm up and down, and another blendspace that moves the arm laterally. I’ve tested both blend spaces, they work as expected. Now I want to add them together. When I take the lateral/ forward blendspace as the base pose, and then the vertical blendspace as the blend pose, then put the weight to 1.0. Then I add the arm in questions shoulder bone, and set blend depth (does anyone actually know what this does, because the internet doesn’t seem to) to 1. All that occurs, is that the vertical animation completely overrides the lateral/ forward blendspace. So I get only the vertical movement from the bone onward that I listed.