Forge die building: why this course exists

Knowledge check

A forge shop has built two nominally identical H13 hammer dies to the same drawing. One ran 220,000 cycles before scrap, the other cracked at 48,000. The steel certificates match, both were vacuum heat treated to 46-48 HRC, both were plasma nitrided by the same vendor to the same case depth, both were polished to the same Ra. Name three places in the build chain where the difference could have been introduced without showing up on any certificate.