Abuse or defects or environment. I’ve, for example, seen one phone which was constantly woken up (technical term in case it sounds odd) because of some event in the wireless signal and that made it use up the battery in a ridiculously short time. It was a combination of the way a network was set up, bad signal quality, and a firmware quirk. Clearly a defect, but hard to say whose. Forcing it to use some mode in the radio via settings circumvented that.