There have been many people shot in police shootouts who were hit in the vitals and continued to fight for quite some time. Miami dade is one of them. Even if the heart or lungs are taken out completely there the possibility of a minute or more of oxygen in the brain to sustain life functions. A very angry grizzly at full sprint wouldn't take too long to close in on 400yards. And remember, the 400m world record for a human is about 43 seconds, a bear at 35mph would cleat that distance in 23. There is no doubt in my mind that it could have gotten to them with plenty left in the tank to kill both.Long range hunting is an increasing trend these days. My guess is the conditions which they zeroed at were not the same as the hunt location... temp, humidity, altitude.
I wasnt there, but if they truly hit vitals I dont see how a bear could travel over 400 yds with a double lung shot.