I' ve never hit a deer in this spot (real or not), and don' t know anybody at home who has either, but if such a thing does exist, I think that Strut and Rut may have the right idea.
My first thought about this " hollow spot" was that it could exist when the animal was at full exhale between breaths. Not sure, but it sounded reasonable in my head. (It was hard to tell with all of the echo.)

...but then there is that whole pleural cavity thing. The lungs do need negative pressure it inflate.