Change of powders, change of techniques, change of ways to measure pressure levels, etc...
The modern powders certainly differ from the older ones. Early DWM stuff was a more slow burning powder, creating a pressure curve that differs from those generated by modern fastburning powders.
The firearms technicians from 100 years ago were not idiots, but they lacked access to equipment and data that is available now. Even as late as the 1970s, companies like Mauser struggled to get the ballistic data they needed.
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