There's a distinction between "Functional Capacity" and "Total Capacity."

I'll use an example from another industry. If a warehouse has 500,000 cubic feet of shelving capacity you could theorize that it could hold 500,000 1 cubic foot sized boxes.

But, you need to allow for ebbs and flows. You need to have enough space available in the same spot at the same time to allow storage for a sudden influx of 40,000 identical widgets. It makes no sense to stuff them here and there everywhere there happens to be an empty cubic foot and they try to find them all back later when an order comes in.

I've read that something is functionally full at about 85% of total capacity.

Still doesn't explain the 2x, but I'm thinking people moving about isn't quite the same formula a boxes sitting on pallets on shelves.