FlightPathOBN:
The battery system is just like all of the other pieces of this aircraft that were designed to the micron, without any regard to constructability and general realistic materiels tolerances.
A discussion in another forum accidentally led me to the following site:
Battery Life and How To Improve It
Among other things, it explains that in addition to the lower chance of thermal runaway in an LiFePO4 cell compared to an LiCoO2 cell, the LiFePo4 has an almost zero volume change during charge/discharge cycling compared to as much as 11% in the LiCoO2 cell (Boeing 787), and that this repeated mechanical stress can damage the separator [leading to a short circuit]!
The fact that the Boeing batteries were unexpectedly being regularly discharged to a lower SOC than planned (because of ground operations error?) may be a critical factor in the reduced cell life.