People just "come back to life" somtimes. It's bizarre, astoundingly uncommon, and we don't understand how it happens, but it's been documented time and again.
The problem, really, is that determining "death" is pretty hard. Lack of pulse? Not breathing? Apparent rigor mortis? Lack of brain function? All pretty good indicators, but you can still be alive with any one or all of them. Even cell death can be a false flag - your system can literally start going to pieces, then just...stop... for some reason, turn around, and recover.
An anecdote, if I may.
My father is a trained medical professional, and once when I was young one of our dogs brought in a opossum, wet with slobber, stiff as a board, smelling of decay, eyes glossy and lightless. We of course knew their famous fake death trick, but my dad was hoping quite strongly that it was still alive when he first started to examine it, since it wasn't really torn up. But he quickly became absolutely convinced the thing was dead as a doornail, the body cold, no pulse, no responses of any kind, even some ants milling about on it - dead quite a while, meaning the dog had merely found it.
He stood up from examining it, looked out across the yard, decided on a place to bury it, turned around and walked fifteen feet into the garage, grabbed a shovel, and came back to find the animal just flat out gone.
My point is, if a perfectly healthy animal can produce all the detectable symptoms of death on purpose well enough to fool even trained medical professionals with decades of experience, what's to stop the body of a less than healthy human from accidentally exhibiting those same detectable symptoms in the course of illness or injury?