If the bullet fragmented on impact, there would have been a sizable wound.
I could be wrong but I respectfully disagree. If it turned into shrapnel it would open up a lot of small spots but not a sizable wound which could account for the terrible blood trail. I have a feeling that buck died somewhere at some time but I've never been able to prove it.
Deer are pretty darn tough critters. Being an avid bow hunter I've seen some amazing things they do because of their will to live.