It's just confirmation bias, a person believes in something and when it appears sometimes to be true it confirms their belief and they ignore all the times it shows not to be true, cancers do suddenly regress for no apparent reason, some people just get lucky, simple coincidence.

That's why belief in horoscopes etc continues to exist, when careful examination and reason, would show that belief to be unwarranted.

It could be that belief in prayer has a placebo effect in a very few cases however we do know that there are many thousands of examples where sufferers have been in receipt of prayers that had no effect whatsoever.

We also know that frequently God is praised for healing somebody and those praising ignore the efforts of the doctors and their treatment.

I wonder how often God is given the credit for a person's survival of a car crash when that credit would be better given to the seat belt/airbag/crumple zone manufacturer or even the MPs that brought in the legislation making them compulsory.