In an article written by Dr. Thomas Smith, one of the authors of that study, I found some additional details about bear attacks where firearms were involved (note: while the bear spray numbers are those from the 20 year study, another article I read indicated the following firearm numbers span a 100 year period):

What position do bear biologists take in this debate? I can’t speak for others, but after studying more than 600 Alaska bear attacks, I've learned:

• In 72 incidents of people using bear spray to defend themselves against aggressive bears in Alaska, 98% were uninjured, and those that were suffered only minor injuries.
• In 300 incidents where people carried and used firearms for protection against aggressive bears in Alaska, 40% were injured or killed, including 23 fatalities and 16 severely injured persons. Another 48 people suffered lesser injuries.

These numbers are consistent with the US Fish & Wildlife findings that:

...since 1992, persons encountering grizzlies and defending themselves with firearms suffer injury about 50% of the time. During the same period, persons defending themselves with pepper spray escaped injury most of the time, and those that were injured experienced shorter duration attacks and less severe injuries.

Based on these observations I see 2 possible explanations. One is that bear spray is more effective than firearms when defending against bear attacks. The other is an overwhelming amount of bears will only commit themselves to attacking those who choose to defend themselves with firearms. The former seems more likely to me.