It's a fact of life that variations in concrete volume can and do occur, but understanding how and why they occur can help make their occurrence less frequent.
Concrete yield is defined as the volume of fresh-mixed concrete produced from a known quantity of ingredients. It is determined by dividing the total batch weight by the measured unit weight of the fresh concrete. Variations in yield are sometimes the result of the contractor's operation. Yield variations also occur because of changes in the mix proportions or tolerances in weighing out the batch. A contractor's operation can create what appears to be a concrete shortage or underyield. Contractors should investigate possible such causes for apparent concrete shortages as: deflection or distortion of wall forms by the pressure of the concrete; irregular subgrades; or forms placed at the wrong elevations or locations.
Laboratory test batches usually are made with the highest specified air and water content to ensure that the required minimum compressive strength is obtained. When the batched concrete has an air and water content that isn't as high as the laboratory test batch, the ingredients don't deliver the same volume that the laboratory test says they should.