I would like to know if he was lucky enough to have missed the ribs.

I have often said that speed will be the best for penetration on skin and hyde due to friction.Speed fights off friction but when it comes to bone hits,momentum is king.(check laws of friction)As speed increases,friction decreases.

I would be willing to bet that at the kinds of speeds this setup was producing that even that light arrow had a decent amount of momentum as well.