Cannibalism was part of the religious rituals. It also happened during time of war (in a ritual context).
That is from the first person account.
All the excuse that were created afterward were made by people far removed from the events, often decades or centuries after the Spanish outlawed the human sacrifice and cannibalism.
Aside from wanting to counter exaggerations, or just wanting to make the Aztecs look better than they were, I think scholars stress the "ritual" nature of cannibalism to indicate the the average downtrodden Mesoamerican Joe or Jane wasn't feasting on human flesh every night.