Difficult subject to determine because of a huge number of variables, including shell variables, more so perhaps than the gun itself. The British had used different criteria for gun penetration testing between 1939 to 1942. British figures generally tend to come out slightly higher. The British generally regarded German armour of poorer quality - hard but brittle with a tendency to crack because of its composition. This could account for the variables, ie a tendency to slightly increase the expected degree of penetration to allow for a suspected inferiority in the armour. This 'predicting' or modifying did sometimes happen in the absence of proper or consistent figures - see my next comments.
The shell though is a massive issue -quality control, powder strength and quality, metal quality etc. American rounds tended to always be slightly worse than expected. The 17 pdr was another classic example. The actual variance in penetration was off the scale - that's providing the round hit. APDS accuracy was so bad a lot of units wouldn't risk using it . But when it hit! - The figures were sometimes astonishing - but also so varied as to be not reliably document-able.