The batteries that showed imbalanced depletion were Energizer Lithiums.
I have ordinary alkalines in Atlas right now. It will be interesting to see if they ran down in a more balanced way.
I have seen this imbalance issue when using Lithiums in series strings in other products: a portable shortwave radio, some motorized window shades. The motorized shades were especially maddening. They held 8 cells, but would fail if one was depleted. After too many runs up the ladder, those shades are gone.
Update, after some reading. Seems that cells have capacity variation due to manufacturing tolerances, and other factors. In series strings, strong cells will beat down weak ones, driving them to early failure. So, no mystery it seems.