7.62mm M59 Calibration loads

What is the difference between the normal M59 Ball loading and the Calibration loading?

Phil–Calibration Loads only differ from the normal load in that they are loaded and tested under VERY strict conditions of temperature and barometric pressure. The loading is done to insure exactly the same results from every cartridge. A table is prepared from the test results that project the change in velocity and chamber pressure at different temperatures and air pressures. This table is sent with each lot of Calibration Rounds. The cartridges are used to calibrate pressure guns at that days temperature and air pressure before testing an unknown lot of ammunition.