why is it that one smoke detector has run out of battery weeks before the other one, when theyve been on the exact same time, and neither has had its alarm triggered, identical models, identical batteries?

Assuming everything is exactly the same you will have to stretch a little. Batteries are know to varry in life span even if they are produced one after another. Climate, the one smoke detector may drain some power over time. The battery gods say it is so.