Angles are very funny things. They are not what you first think they are. Did the arrow catch a rib? That can be enough for a deflection suprizingly. A deer I took last year clearly showed my arrow skidding at least 4 inches UP a rip before it hit the spine (w/close to 70flbs of KE), and that was with a significant downward angle on impact.

sometime you just have to scratch your head, and be glad of a happy ending!