Maybe he looked so chewed up that they overlooked him to engage a second, still-mobile target?
Certainly possible. From what I can tell from reading the different accounts, the bad guys stopped to reload. If that's accurate, they might have fired 100+ rounds amongst the 4 of them and made 27 hits. So 4 guys with rifles firing, let's say 60-120 rounds total, couldn't kill one guy at almost point-blank range.
Chief Day took them out with a 15+1 9mm and presumably didn't reload the pistol. That level of training gets down to the subconscious.