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.
If this is how the seller of the cord of firewood sells the cord to you, how much did you pay for the wood and how much did you pay for the air and dirt that is usually associated with a pile of firewood. The dirt factor can be overlooked if the stacked wood is placed on level ground. But even in a neatly stacked pile there will be air spaces.
In addition to air and dirt diminishing the amount of actual amount of wood in the pile, buying green wood adds additional loss to the volume due to shrinkage.
A seller of firewood is doing nothing wrong by selling the firewood by the above mentioned dimensions, but in all fairness they should increase the dimensions to accommodate shrinkage and loss of volume due to air space.
Since the shrinkage of wood varies depending on the species of the wood 8% is used as the factor for shrinkage. (It could be a bit less, or much more depending on the wood species.)
For air space in your pile of wood the factor is taken at 25 %. (It could be a bit less, or more depending on how neatly the pieces fit together in your pile of wood.)