Well duh, shootemup. Of course bullet companies only test their own products. Alliant doesn't test Hodgdon powders, either.
When powder makers test for load data, they will use a selection of bullets in a given weight and publish load data for the bullet that generated the highest pressures. The theory is that other bullets will generate lower pressures with that load data, and therefore be safe. However, as there can be as much as 10,000 psi difference between the highest and lowest pressure spread with different bullets (even of the same weight), you might be getting significantly less performance than possible.
Bullets vary quite a bit, but powders don't. Acme 3127 powder used by bullet company A to do their tests is going to be very close to the lot of Acme 3127 used by bullet company B to do their tests.
Bullet company data should be your primary source. Crosscheck with powder company data. If there's a major discrepancy, verify with an independent tester (Lyman).
Finally, the Lee book is fine, but they do NO testing. they merely copy data f-r-o-m others, both bullet and powder makers. Therefore, Lee is not a suitable independent source.
BINGO! f-r-o-m did it! Thanks Mike.