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Along with the already mentioned slight change to the powder, and the difference in pressure measurement, AND the liability factor, consider also the test guns used.
Every gun is an individual, to a greater or lesser degree. And the components used make a huge difference. Note the firearm used, the cases used, the primer, and bullet. For generations, the standard advice has always been, that when any change is made to any component, drop at least 10% and then work back up.
Book max is max with the ammo tested, in the gun tested. Period! It is a guideline, not a rule. Your gun/ammo combination may take a bit more, or may not be able to safely take as much!
A safe load with commercial brass might pop primers with GI brass. You simply cannot tell just by looking, and reading it out of a book or off the net only tells you what they got, with what they used.
I have been using Unique since the 1970s, and the loads I used then, in the guns I have used since, have been safe. But always, when getting close to top end, I develope loads for the indiviual guns, and don't use them in others, without working up.
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All else being equal (and it almost never is) bigger bullets tend to work better.
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