This article helps to explain some of the differences, as well as the criteria used to establish brain death.
I would bet that none of those articles you linked were bona fide brain death cases where the patient was examined by two physicians separately as is legally required before the patient can be pronounced dead. Given that they were published in the mainstream news, they probably contain a pretty thick helping of sensationalism. One article even says that the teen was put into a drug-induced coma--meaning that no one ever believed he was dead (even though they did not think he would survive). The woman in another article was never examined for brain death. The boy in the Today article supposedly had a PET scan that showed no activity--but that is not the type of examination that would establish brain death.
As far as I am aware, only one person has come back from the dead, and that happened about 2,000 years ago. It was also clearly a case of Divine Intervention.
One of Jahi’s doctors does not agree with the diagnosis. For an innocent life, I’ll side with the dissenting vote.
Everybody forgets poor old Lazarus.....