Excellent question. That is because you are looking at the 12fpe column. In that power level for each caliber, the .22 gets more shots from a fill do to the weight of the pellet being more. A heavier pellet will achieve the 12fpe at a relatively slower speed, which requires less air. (Around 583 for a 15.89gr) The .177 is relatively much lighter and will require a higher speed to achieve the 12fpe, therefore it will use up more air that the .22. (Around 800fps for a 8.44)