As I understand it the lead & copper crushers were calibrated by use of a piston driven by a crank which crushed them while measuring the pressure necessary to crush them to various lengths. This was near to a static measurement of pressure. The duration of the peak pressure in the chamber is of such a short duration these crushers simply were not collapsed to the true extent of the pressure. Lead & Copper would not necessarily compress at the same rate so pressures would not read identical for the two components. The PE devices are fast enough to actually record the true pressure per square inch, thus if a difference is recorded between the methods the PE pressure will always be higher than the crusher measurements. I was just a lowly Machinist & can understand this.
