Re: IV vs Barrel Pressure
You've already touched on the answer here, but in short, no, you're talking about an entirely different set of circumstances and variables. Even if you're using the "same" components, they're from a different lot than those used in the manual. This can mean some substantial differences when speaking of powders, and can vary substantially when we're talking about different lots of the same primer. Cases vary from one run to another, even within the same make of brass. Add to that the individual history of the brass, i.e. how it's prepped, its hardness, the neck tension exerted on the bullet, etc.. Add to that the fact that bullets may vary as much as .0005" or so from one run to another with many manufacturers, and there may even be differences in jacket thicknesses or core hardness (alloy). I've seen all these factors in play during various production runs over the years, and can assure you that they do exist. And finally, we have the rifles themselves. Even if you were to use the very same ammo, actually loaded by the lab that did the manual, it's going to show different results when used in another firearm. Different throating, variable in chamber dimensions, bore/groove diameter variations, and even bore finish are all going to play a role in the final results.
You already touched on the right answer here; the guns vary considertably, as do the components, even when we attempt to perfectly match what's in the manuals. They are a guide, nothing more, and need to be regarded as such. Personally, I've always thought it best to view any reloading manual as a simple report, essentially saying, "we tired this particular combination in our rifle, and this is the result we observed." Follow that up with the ever-present caution that "your mileage WILL vary" and you're a long way down the road in understanding how to read a manual.
Hope that helps,