Many aspects of medicine study the diversity of the human body from the top of the bell curve downward, which is of course the rational way to do it, but they're still missing the infrequent phenomena that may arise as the result of genetic variations, diet, and so forth. (The existence of those variations is what makes evolution possible.) Every so often a combination of those factors may result in remarkable ability of a person to heal from third degree burns - perhaps not entirely, but still remarkably enough to inspire the talk of "miracles", which tend to get amplified each time the story is told.
This is yet another reason why all patients must be treated individually, and must be allowed to act in their self-interest, rather than being forced by the guns of the state!