Those things happen sometimes.
A few years ago, I shot a nice doe with a Muzzy MX-4. The deer went face first into the ground, got up, ran a bit, went face first again, and ran away through the woods on her face. I sat there dumbfounded, knowing, or at least thinking, that the shot had looked good. I walked the trail parallel to where she ran, then cut over and found a little blood. Long story short, I found her, she had run over 100 yards, with a perfect broadside double lung hit. After looking her over, I found that my arrow had just clipped the tendon on the back of the front leg going in, and had clipped the one on the other side going out. That is why she kept falling. I can only deduce that from her running, chest on the ground, that the collapsed legs had come up and covered the entrance and exit wounds, and held in the blood and air from escaping.
Sounds weird, probably a one-in-a-million chance, but anything can happen.
Glad to see that unlike most, you didn't blame the broadhead.
TB