First off, when I say "working limit" what I'm talking about is the "hottest" load level BEFORE pressure signs appear.
now the question why do guns have different "working" limits?
Because everything made has a range of tolerances that are acceptable, and how these react in combination give us the result. When the stars line up one way, we get one result. When it's a different way, then there's a different result.
Minute differences can add up to make a noticeably different result. Usually they don't, but sometimes, they do. All the factors in the design and construction of both the gun and the ammunition, AND their relationship with each other are the reason one gun and ammo combination will show some pressure sign, and a different gun with the same ammo may not.
A revolver with a rough chamber will give sticky extraction at a lower pressure than one with a properly polished chamber. This is an example of the quality of a gun's manufacture imposing a "limit" different between two different guns.
A gun with thinner chamber walls will "give" more under pressure than one with thicker walls which support the case better. This is an example of the gun's design imposing a limit.
What makes a Model 19 get "sticky" at one level and another firearm like a Ruger Blackhawk "work" at a much higher powder level? Both are quality firearms made with care.
simply that they are different designs, AND different individual guns. There is a level that will stick cases in a Ruger, there is a level that will do it in everything. What that level is, exactly, can be different from gun to gun. Most of the time it is so close to the same we don't see it, or we see only a small variation. Chronograph the same load from more than one gun (same barrel length), and see if you get exactly the same fps from all of them. You won't. You'll get a range of results.
In a really poor analogy (but the only thing I can think of at the moment), you know how the microwave directions on some foods say X min on high, but then also say how microwaves vary and you may have to adjust the heating time? Guns are KIND of like that. They vary.