New growers treat casing as a second layer of food. It is not. Compost carries the carbon and nitrogen the mycelium eats. Casing carries almost none: it holds water, lets carbon dioxide escape, and its surface is where pins form.
So a farm can have good compost and no crop when the casing is too dry, too wet or diseased. And good casing gives a poor crop when the compost was spawned with ammonia still in it. Yield is counted per 100 kg of compost because compost is the input everything else scales from.