Gotta read the whole description of the test unit used for the books load development.
Primer, case, chamber, the leade (distance the bullet jumps before engaging the rifling) the actual bore diameter of the barrel, the actual diameter of the chamber reamer used, etc., all affect the results.
Add into that, the comment about liability (if you read through some older load books, the hot loads were really hot) and generally different test methodologies from company to company....Pressures calculated, pressures tested by Peizo transducer, or copper crusher test, ...
Anyways, read through as many load books (current ones) as you can get hold of, and pay a lot of attention to the stuff up front about load development, as that really is the good info.
Learn what the signs are of overpressure loads, as your rifle may well show them at less than max listed, and start your development somewhere near the middle of the bunch, rather than just loading up a whack to the book specs, on the off chance that it will shoot well.
Ask lots of questions. Pay attention when you are loading, eh!
Cheers
Trev