I agree that 20-30 seconds with both lungs gone is not typical, but not unseen. I also agree with the comments on bow kills, I believe with the double lung lung shot bear die quicker with broadheads than bullets. Archery folks tend to avoid the big bones, wait for the position to be right more often than rifle shooters, and usually have a good exit hole. Back in the olden days when bait and hounds were legal here, too many late night calls asking to bring a dog to help find a bear, kind of shaped my thoughts on what it takes to avoid "atypical" results. I know of one bait set that 5 big bear were shot from over the years, and only one retrieved. When I mull it all around best "guess" is failing to get both lungs due to bullet construction. I don't have a preference shoulder, or lungs. I do have a preference for packing a rifle that allows me options, and works when things don't go just right.
Thanks for the bear picture, what are you packing there? I agree-those bullets that punch through without doing much damage are the ones that give the atypical 20-30 seconds, and use it to get someplace that will give a guy second thoughts about following.