I have the idea that GPU performance is affected by the apps used. Of course simultaneous use will affect performance, but maybe not all apps leave the GPU in the state they found it in.

I have for example Marvelous Designer GPU simulation and Poser Superfly both using (optionally) the cores in my 1070GTX. I have the impression there is a difference depending on the sequence I use them in. I can imagine same holds for games and tools like Blender.