I think we fail to appreciate how much the tiny difference in volume between a SAAMI minimum spec chamber and a maximum spec chamber can make in the resulting pressure and MV. For liability reasons, the bullet manufacturers would be crazy if they didn't use proof barrels that leaned to minimum spec side, and the gun makers would be crazy if they didn't favor building max spec chambers for the same reason. Imagine the legal dustup if Hornady developed a commercial load using a max spec chamber and Savage built a rifle around it that used a minimum spec chamber.
Then there's the fact that different lots of powder tend to have slightly different levels of 'potency.' And gunpowder likes to absorb moisture. So unless you live in Arizona, and unless you just broke the seal on that bottle of powder you're loading from, a given volume of powder is going to weigh a little more, which means a given charge weight has less 'potency.'
And it doesn't help matters that the biggest-selling brand of chronograph also happens to be the one with the most marginal performance. Caveat emptor.
