I know no more about these incidents than was in the original article, but I can think of two scenarios where bear spray could be employed correctly followed by using a firearm.
If the bear stops after being sprayed at the appropriate range, the hunter moves away, and then the bear continues to attack, it's possible that a firearm could then be employed. Similarly, if there are two hunters (as in one of the incidents) and one uses bear spray correctly while the other is ready to shoot, that could be an explanation.
Like I said, these are hypothetical explanations.