NOT THE LAWYERS (except maybe sometimes).
The SAAMI standards, which you can download free from their web site, have not changed since 1992 for rife and 1993 for handguns. When you read the standard, for every cartridge there are standard bullet weights with standard velocities listed. Those numbers are also unchanged since those dates.
Neither have pressure standards changed since those dates, except to add more cartridges than were listed back then. The method of measuring them has moved away from the copper crusher and toward the conformal piezoelectric transducer. The copper crusher is a poor measurement system for absolute accuracy and it disagrees with the transducer result in most instances. It is also non-linear at the higher pressures, tending to under-report them significantly.
For example, if you take the same lot of 357 Magnum pressure reference ammunition and fire a ten round sample for average pressure in a SAAMI copper crusher, it reads the assessed pressure of the reference ammunition, with about a 25% range of disagreement among different copper crushers operated by different people. But if we suppose it read 45,000 psi, if you put sample rounds from that exact same lot of reference ammunition into a SAAMI conformal piezoelectric transducer, it would read 35,000 psi with about 12% range of disagreement among different transducers. If you had put the same lot of reference ammunition into a CIP copper crusher (now obsolete) it would have read 3200 bar (≈46,400 psi; they never adopted the CUP or its equivalent). If you put it into a CIP channel piezoelectric transducer, it reads 3000 bar (≈43,500 psi). CIP claims their channel transducer system is accurate within 2% of absolute. I have no idea how they arrived at that conclusion. It is likely a statistical result based on what is actually consistency, but without reading how they arrived at that conclusion, I don't really know.
So, does anyone believe we actually know what the absolute pressure of a 357 Magnum is? Fortunately, we don't have to. The way the reference ammunition concept works is that one SAAMI member manufacturer, in this case Remington, is responsible for producing reference ammunition for the whole industry to the same standard. Everyone who measures pressure fires ten reference loads, and, regardless of what average pressure their instrument reads with those loads, they consider it to be whatever the assessed pressure of the reference load is. If their reading falls outside the inclusive limits for the reference load, they scale their reading to match and scale all their other readings that day with that same adjustment factor. So the instruments don't have to be absolutely accurate, they just have to be repeatable.
From the above, you can see that when load data are developed in SAAMI standard pressure measuring equipment, as is the case for Hodgdon and many of the loads in Lyman's manual, for example, that reference loads for the round were fired and nothing about the pressures or velocities produced has changed since 1992 or 1993 except for the addition of newer chamberings. No lawyers involved. Indeed, when I questioned Hodgdon about one of there results on time, they protested that they had the ballistic technicians signed data sheet on record confirming the results. So, that's their main line of legal defense: they conform to the SAAMI standard, and that, as I said, hasn't changed in over 20 years.
It is only where you have manuals (I'm thinking Sierra and Hornady here) in which all the load data was developed in commercial firearms and not in SAAMI pressure/velocity barrel guns, that you see reduced level loads (significantly below SAAMI standard maximums). Here, I believe, an extra margin of caution is being applied because they don't have the signed sheets showing they comply with the SAAMI standard. Also, they are presumably reading brass and primers for pressure, and as Denton Bramwell showed, two brass cases from the same lot with the same load history can disagree about pressure by almost 50%. So this is a situation where the lawyer's may have made a recommendation, or the company just decided on its own that the loads were where they were comfortable with them, with the idea of avoiding being sued somewhere in mind.
There is also the odd case of Speer's manual. Like Sierra and Hornady, they develop the loads in standard firearms, but they then send the maximum loads to Alliant for subsequent pressure testing to confirm they don't exceed SAAMI maximum. That's their legal defense. The starting and middle load pressures in the Speer manual appear to be estimates based on the maximum rather than actual measurements. But the final effect is that Speer has a few loads that are warmer than Sierra or Hornady.
A case where maybe the lawyers maybe should have been involved:
Here's a funny exception to the lowering of charges for you. Check out Western's load data for 223 Remington and 5.56×45 NATO. The powder charges are heavier for the latter. That should not be.
If you take a 5.56×45 U.S. Military reference load and measure it in a conformal piezo transducer (mil standard SCATP 5.56), it measures 55,000 psi. This is the standard the U.S. military produces 5.56 NATO cartridges to; a measured 55,000 psi. If you put that same exact lot of reference ammunition into a NATO channel piezo transducer (EVPAT standard) it measures 4300 bar (62,366 psi). Same round. Same absolute pressure. All the difference in the measured values is due to differences in the measuring systems used, and is not due to an actual difference in absolute pressure.
Well, Western's Ramshot and Accurate load data list 223 Remington and 5.56×45 NATO separately, and loads the latter to the 62,366 psi limit using a conformal piezo transducer. That is obvious from the higher charge weights listed for 5.56×45, but I also have that from the horse's mouth. I spoke with one of their technicians about this error and he confirmed the conformal transducer is what they used for both 223 Remington and 5.56×45 load development. Not valid, which he didn't know. So all that data they list for 5.56×45 is 13% above the pressure either NATO or the US military actually load 5.56 NATO ammunition to. If it measures 62,366 on a conformal piezo transducer, it would like register about 70,700 psi on a NATO channel transducer.
That is still below the proof load range (starts at 69,500 psi by conformal transducer), and it's something many bolt guns will handle just fine because the same actions and barrel diameters also handle the wider .308's. But its enough higher than standard that increased throat erosion rates can be expected and I would not want to use those loads in the AR and Mini14 gas systems because of the extra wear and tear.
of shooting one of the .357 Mag. snubbies like the M60 and frankly, have no desire even with the lower powered loads now sold as full power.