While I agree that some manuals may have become more conservative due to liability fears, other, more genuine reasons include:
i. The powder branded XYZ today is often not the same powder as in yesteryear. Companies change their formulas over time but retain the product name for marketing reasons. The new XYZ will be somewhat similar to the old XYZ, but not necessarily identical. Often the branding is in the hands of a distributor, such as Hodgdon, they will issue a general specification for a particular powder and then tender out the production to actual powder manufacturers. Your batch of XYZ may not have been made by the same company as the previous batch.
ii. Pressure measuring technology has changed significantly. In the old days most pressure was estimated from copper crush technology. Now a days digital technology and transducers are the go to. In some cases some cartridges weren't tested but rather interpolated from similar cartridges and/or calculations based on case volume, burn rate etc.
iii. Worse still, in some manuals, loads weren't scientifically tested at all. They were based on the author's, and his mates, best guess using subjective judgements from the look of primers, expansion of case heads, ease of extraction etc.
