If you have an exit hole on the other side of the ribcage, it's doubtful the arrow backed out. Rather, the arrow just stuck out a bit on the other side and the fletch end broke off and the broadhead end pulled through the offside. This is indeed a sign that you are getting poor penetration if you're shooting over 55lbs.
Arrows will back up, this is usually when you hit the offside shoulder or leg.(as mentioned above) It does happen fast enough that you can't really see it. However, if you looked at the inside of the ribcage and the offside shoulder you'd see evidence of the arrow hitting there. (if you process your own deer)
Seriously, bare shaft shoot your bow and shoot it through paper. Those things aren't up to conjecture.
