To go back to the original question that Zog posted, others stated correctly that the components used in manufacturing result in the differences. I agree that this is a great contributing factor to being difficult to series rate different breakers.
In the context of coordination studies, that is what makes studies of existing facilities so challenging (interesting). At times the adjustments could be made to the settings of the OCPD to make the coordination selective to an acceptable degree, although there's usually some degree of miscoordination somewhere in the system. Other times the IT curves just don't work out and replacement is necessary.