I doubt that the methods used by Speer to determine the level of those loads has varied significantly over those years. My reason of choice is that the lawyers have a whole lot to do with those numbers.
I believe up until the #10 manual Speer used the case head expantion method of judging pressure. They then went to a crusher barrel, and now are useing a piezo system. There is a world of difference in the accuracy of the new pressure measuring systems compared to the other two.
Case head measurement only tells one the case expanded to a certain size or comparably to another case. How much pressure it took to do this isn`t known, but "estimated". The case is the weakest link and how bad it is stressed is a indicator of how safe a load is or isn`t. Loose primer pockets after one loading is a good indictor the load is too hot for example, but still doesn`t tell us the psi of the load.
Crushers rely on uniform pellets, accurate referance charts, and the user to accurately measure the pellet with hand tools. Again, accual pressure isn`t measured, just its effect. Done right by though experianced techs it does offer much more useful and accurate information. Pressure can be quite closely estimated when done right.
The Piezo system gives the best, most accurate result of the three. Piezo tells us just what the pressure is. It also better exposes pressure swings that may be possibly dangerous, because resolution is half of the crusher systems. Pressure variation can cause the data writers to lower the max load because of fear the pressure may spike in rare cases.
I have no doubt the lawyers are pushing the writers to stay conservative, that`s their job. I doubt they are the cause of data being reduced by the scope it appears to be in all the various books. I have heard more then once both in magazines and on the web that the reason for the reduction in loads for the 7mm Remington mag is the scope of pressure swings found in the cartridge by the factories as they switched over to Piezo. The 243 win I also have heard is a good canidate for spikes. Data in both cases has been sharply dropped lately.