One has lower capacity than the other (probably due to small differences when they came off the manufacturing line resulting in them aging differently).

Or maybe the difference in distance between the AirPods and the signal source is just enough that one has to frequency hop or request retransmission of corrupted packets more often than the other, resulting in higher power consumption (I’m almost completely ignorant of the Bluetooth protocol so I don’t actually know how it handles dropped/bad packets)

Bad contacts in the case, so one isn't charging as well? One got left in a hot / cold place for a while, and the battery chemistry changed? One of the cells in one of the batteries went bad?