Heres what I would think happened ... you hit in these red lines - somehow, someway, and theres where your heavy bone is. You cut through meat, your head hit that bone and curled up - thus your arrow had 3 3/4" penetration or so.

What does everyone else think ? A higher hit in the flat of the shoulder blade, with a 63# bow, should have punched through it (not as heavy and not round like the lower leg bone is)