I have to agree about hitting higher then you think. Shot placement is high (IMHO) to start with. If the deer was looking right at you on high alert, he could get lower pretty quick. This would put the shot high enough to be unrecoverable or even still a live animal. Glad to see that you did not blame the broadhead or equipment for a marginal shot.
Best thing to do is learn from this experience and become a better bow hunter. As everyone on this forum has seen, deer are tough. He could still be out there walking around, why aren't you?