Here is part of UncleNicks explanation, sorry no link just saved text.
If you look through military ammo specs from before the mid-90's, you find military copper crusher numbers reported as "psi" at the same time a commercial maker would have reported them as "CUP" (I don't know why "CUP" gets caps while "psi" gets lower case, but that's how SAAMI publishes them). The SAAMI conformal piezo spec is 62,000 psi MAP (Maximum Average Pressure). I'll have to check, but I recall SAAMI MAP allows up to 4% deviation from average, so 64,480 psi would be the allowed extreme spread in SAAMI compliant ammo.
The NATO allies use 415 MPa, which the CIP uses for both 7.62 NATO and .308 Winchester. That converts to 60,191 psi, but the CIP measuring system sets the sample point further forward in the case, so it sees a bit of the pressure drop at the bullet base that occurs when the bullet starts moving. That makes them tend to read about 2,000 psi lower than the SAAMI setup does in this pressure range. So the same ammo would measure closer to 62,000 psi in SAAMI equipment.