The human body is an amazing thing that we still do not quite know how it works. We are able to explain a lot with our advanced scientific methods, but there is still so much that we cannot explain. Whether it is intervention by a higher power or abnormalities that cause above normal function of the body, no one will ever know.
There is no denying that, for a select few individuals, the normal death sentencing effects do not work. May people have survived multiple gun shot wounds or even worse, only to go on to live a normal life and function the same way as they did before the incident happened.
If such people existed, medical research would probably have a genetic explanation for why this happens. Every one has physical differences and some of these may allow certain people to survive deadly injuries better, but these differences are in the range of normal body functioning, and not anything genetic. Simply having better health would not be a strong basis for this assumption.
I don't think that there is any credible evidence to show that such kinds of people as the ones asked in this question actually do exist. There are outliers in all aspects of life, and people who survive gunshot wounds are just another group of these types of people out there.