Well under normal circumstances there usually isn't much if any void.The chest cavity is somewhat pressurized and the lungs go pretty much up to the top of the cavity. But, here is my theory: This particular buck jumped the string, and if he had exhaled while doing so which would be normal, it is likely that the deflated lungs pushed down slightly in the cavity while he was on the downward stroke of his jump.The arrow barely entered the chest cavity passing just below the spine. It came so close infact that according to my brother-in-law the deer initially fell immediately to the ground like a spine hit buck would, but then jumped right back up. One lung was a tiny bit pale at the top, but no cut or hemorage was found. Also keep in mind as I had stated that the top shot in that picture is deceiving as that is the exit hole. The entrance hole on the other side is much higher. More often than not, that shot would have killed that deer. I wonder if he had used a broadhead with a 2" wide cut like mine on the bottom if that would have brought him down for good. In any case we were both thrilled at the fact he survived and I was presented with another chance at him.