Because clearly they fucked something up. He was still getting oxygen somehow. I’m guessing they didn’t have the nitrogen flow high enough so he was still getting some oxygen.
It could have also just been agonal gasping which can last over an hour even after the person is already dead. It’s fairly common for people to see that and say the person is still breathing even though that person has already been dead for a while. It also happens with heart attacks and it frequently leads to ems having to explain to family members why there is no hope of resuscitation even though to them it looks like the person is still “breathing”.