In preparing tarage tables a dead weight or constant force is applied to the crusher.

When firing, the pressure is very brief. At the pressures involved the crusher behaves a fluid, but a very viscous fluid, it takes time to deform. Since the peak pressure is so brief, the crusher does not have time to deform to the extent that it does under static prssure.

This is the main reason for the CUP reading being lower than a piezo reading. There are also other errors do to various causes but that is the principle error.