Usable vs nominal capacity
Nominal is all the energy that physically fits; usable is what the car lets you use, keeping a reserve buffer to protect the battery.
Makers keep a buffer at the top and bottom of the battery β they don't let you reach a true 0% or 100% β because charging to the brim or draining it fully ages it faster. That buffer is the gap between nominal and usable capacity.
For almost everything a driver cares about β range, comparisons, consumption math β usable is what rules. When you see two kWh figures for the same model, that's why.