Having not seen the photos I will say this:
Internal anatomy isn't consistent. Blade rotation and path is always somewhat random. There is always a chance for a 'well hit' animal to escape and live, even when the hit appears letter-perfect. It's our nature to find an explanation or some type of logic, but it defies logic. I have personally seen deer survive chest hits with total penetration from a sharp broadhead. One guy killed the previously-hit buck later and proved it. This kind of thing is why we read about people who take 3 gunshot wounds in the chest and live, while someone gets one in the gut and dies in 20 minutes. I've shot deer squarely through the abdomen/intestines and found them dead quickly in a couple cases.
You might make the same hit on 20 more bears and kill every one of them cleanly. Don't let one fluke occurrence mess with your mind. This is is bowhunting and it's not 100% even when it looks that way.