Most people who use firearms are poorly trained and don't have appropriate firearms for killing bears. So they injure and aggravate the bear and it now feels it has to kill the human. Whether this hypothesis is true can be tested by comparing the number of injuries among general people with guns and among people who are well trained and use appropriate guns.
You and JohnN both raise a good point; injuring a bear that was bluffing can definitely cause an attack / mauling that might not have occurred otherwise.
I can also see inexperience being a factor as you suggest, but good shots don't appear to guarantee an immediate end to an attack. For example,
Scott Oberlitner and a buddy defended themselves against a bear attack, him with a .375 H&H Magnum rifle. Each man fired at the bear before it came into contact with Oberlitner, but with less than ideal results even though both shots were good:
"... both of the bullets fired at the start of the attack mortally wounded the bear, but because those bullets failed to hit any bone, they did nothing to slow the bear's attack. The force of the last shot rolled the bear off Oberlitner."Based on my limited reading, I think there are things that can be done to increase the effectiveness of a firearm as a defence tool against bear attacks, including:
- Having your firearm in hand
- Having your firearm loaded with a round chambered
- Being able to disengage the safety smoothly and intuitively under pressure
- Don't try to scope the bear
However, I'm not convinced firearms can be brought up to the effectiveness of bear spray.