

If I remember correctly, it comes from measuring volume coming to/from large bodies of water where surface area (acres) and depth changes (feet) are easier to measure and there is little reason to do unnecessary conversion to other, more common, units of volume for industry-specific purposes, especially if others outside the industry rarely see or care about such values.
They use air pockets but they are of the stiffer variety that are fitted around each drive, not the super cheap and thin air pockets that just get jammed in to fill the box.