RC is correct that Lee compiles data from other reliable sources and that is why the cost of the book is minimal. That is NOT to say the information is incorrect or sloppy; it is not.
The inconsistancy you've found is well within the expected norms. Fact is, no change we can possibly make to any book data has as much effect as the weapon it was fired in. Each manual maker, be it from powder, bullet makers or neither, lie. They all tell us exactly what they got for the listed charges IN THEIR GUNS, but each gun changes the results even if everything else is duplicated. Our guns are different from any of theirs so our results will also be different too. That's why ALL reloading manuals tell us to start low and slowly work up to book max unless excess pressure signs tell us to stop earlier; it seems a lot of people have trouble understanding that caution and just expect all book data to be spot on even tho it's clear that cannot possibly be!
Bottom line, our reloading safety lies in our own hands, not a special book or data from any other source.