Do Powders Change?
Yes, once in a while. But powder makers go to great lengths to ensure uniformity from batch to batch. But there are exceptions. Back in the 1970s we had identified THREE different burning rate to H110 powder. This, of course, affected the max load amount.
What changes more often (and sometimes with each new edition of the manual) is the specific combination of test gun, bullet, case, primer, and powder.
Each component has an effect on the final pressure generated, and on the velocity that pressure produces. Some guns are "faster" than others. Some brass is thicker. Some bullets have different levels of resistance to the rifling, etc. Each combination of components and gun is an individual.
That is why the reloading manuals are guidelines, not hard and fast rules.
All you can absolutely count on is that they are accurately reporting the results that that they got with what they used. I have had guns that would take loads over "book max" without showing any signs of excess pressure. Likewise I have seen guns that showed classic signs of excess pressure well before reaching book max.
EVERY manual includes the advice to start low, and cautiously work up. And they will tell you to start the process all over if you change a single component.
While you should not expect identical results (they are nice when they happen, but should not be expected) getting something close is usually the case. Your gun likely will be a few fps different from the test gun. Either high or low, this is perfectly normal.
Also, the max loads listed in the manuals are NOT the point at which the gun is likely to blow up. They are the max working load (for the components, including the individual gun tested). There is a safety margin. However, since that safety margin includes the individual gun used, it can, and does change, sometimes radically, when a different gun is used.
Ruger New Vaqueros are not as strong as Vaqueros, or Blackhawks built on the large frame. New Vaqueros are generally regarded as just as strong as Colt SAAs and clones. Now, while those guns aren't bank vault doors, neither are they eggshells.
You can get a 250gr bullet up to 1000fps in any of them, safely. Its a top end load for a Colt, or the NEW Vaquero, but still safe. In a larger gun, that load is not the max.