On a very steep angle if the BH catches the shoulder blade it may follow it and never penetrate the body cavity. Early in my archery hunting I did exactly that on a deer that was too close and walking away. Being young and foolish, I thought I could put an arrow between the shoulders, breaking the spine and not have a tracking job. I was a bit left and the BH (a 3 blade Snuffer) hit the top of the left shoulder blade and slid right down the outside of it.