PA is exactly correct, components have changed over the years. The old H4831 listed in 30-year-old manuals was all surplus powder from various sources - today it is all recently manufactured and some lots vary from the old material. Primers have changed in some brands (DOT/EPA regulations), and cases made 30 years ago often have a different internal capacity than those of recent manufacture.
Perhaps the biggest difference is that old manuals usually did not use pressure measuring equipment when developing data, just case head expansion, primer appearance, etc. These indicators are known to give false results, with very high pressures possible. That is why some data from 30 years ago is lower than today's - it
was a guess, now it
is factual maximum pressure. Lawyers had little to do with it, engineers did. Strange how some believe that if the gun doesn't blow up then the data is safe - cartridge and firearm design parameters are ignored in the pursuit of higher velocities.....
The old manuals are great references and historical documents and I have dozens - but their utility for
good loading data is limited.
.