Daniel -
If you'll study the manuals closely you'll probably see that for the same type of powder, the cartridge with the shorter COL also calls for less powder.
For a given bullet, a shorter cartridge means the bullet is seated deeper in the case. The deeper the bullet is seated, the smaller the volume remaining inside the case. A smaller volume requires less powder to burn to achieve the same chamber pressure. Follow?
The volume inside a 9mm case is tiny as compared to something like a 38 Spl. So on a 9mm, small volume changes means big swings in chamber pressures.