Actually, it's not that hard to imagine. Suppose you had a huge block of solid wood that measures exactly 128 cu ft. If you cut it into log lengths and split it down, you create voids from the sawdust and irregularities. You would have to pack that all back down to create the huge block again, and that is an impossible mission. In general, the smaller the splits that you make, the less wood that a cord contains.
But, deliver splits too large, and the customer will complain, "My fireplace won't burn those big chunks."
And, deliver splits too small and the customer will complain, "I want more sleepers for my stove, not kindling."