When you buy a cord of firewood the cord should fit within an area of 8 feet long x 4 feet high x 4 feet wide or any combination of those measurements that add up to 128 cubic feet. 8' x 4' x 4 ' = 128 cubic feet.
On top of paying for the wood you pay for air. Depending on the way it is stacked your pile of wood could have as much as 25% of airspace.
In addition to that, if your cord of wood is made up of green wood, by the time it is seasoned and ready for use your pile will have shrunk. The shrinkage will be most noticeable in the height of the pile.
But that is what it is and a cord is a measurement for firewood that includes the wood and air.