Miss Nancy, the volumetric shadow is what you can see (the darker areas between the spheres and their ground shadows) both in my image and in yours. That isn't flat shadow on the backdrop or the ground, since the spheres are not shadowing those areas. It's shadow on the volume.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)