Why does the Tesla battery module capacity not work out in calculation?
The batteries simply last longer, if they are not used up to the full capacity. Maybe the battery pack charge level is not allowed to go fully empty and they leave it 3% charged, and maybe the pack is also not fully charged but only up to 97%. The pack also has to provide a certain capacity over a temperature range, charge cycles, and battery age, so that is why the de-rating is needed.