There are so many ifs and unknowns in this story it's impossible to determine what happened unless the animal is recovered. If you happen to see it or get pictures of it later (assuming it survives) that may give you some additional insight into what may have gone wrong. I'll say this much - quite often what the archer thinks they see, and what actually happens, are two entirely different things. While it's not impossible for an arrow with that type of setup to glance off the ribs at a quartering-away angle, it's pretty unlikely. A true mid-rib hit with your setup should kill a deer in pretty short order. Personally, I think you slid it into the back of his near-side shoulder. Of course, I'm just speculating based on what I've read here. I will also end with the statement that I don't particularly light a lightweight CX arrow shooting 60 lbs and a 2" mechanical up front. While I don't think a super-heavy arrow and 200 grain single bevel head is necessary either. I do prefer something in the 450-500 grain range for whitetails. At 60 lbs you're probably shooting a 400 spine arrow, which is going to be a touch on the light side anyways.