Every single time the battery is drained / charged, the BMS adjusts this value according to what it can sense : current that went though its shunt, soc, etc. Using an algorithm that we don't have access to, it tries to predict the available capacity according to the current voltage of the battery.
The software that runs on the BMS cleary starts with some static values. i.e. Tesla has programmed the BMS on a brand new 85D to assume that the pack has 85kWh of full capacity including a 4kWh buffer (81kWh usable).
After a few cycles, the BMS adjusts this value (down) because the pack doesn't have the programmed capacity.. so you see a lower value.
That 84 you're seing there is on 85D with 300km on the odometer. Let's assume that car came 90% charged from the factory... guy drained it while driving 300km (less than a full charge) then did a 100% range charge. I.e. BMS has only seen a partial cycle. It doesn't know the actual voltage curve of that specific pack when it hits around 5%... so it must assume that the pack behaves according to some sort of lookup table it has.
My point is.. until you've done a couple of full cycles (100-0-100), the BMS can't know for sur how much power it can get out of the pack... and at 300km on the odometer, no way this BMS has seen a single 100-0-100.