I would say that it depends on the manufacturer. For example, I believe Berger is using Bryan's BC's as their listed BC's so they are identical. On the other hand, Bryan's BC's are significantly different from, say, Nosler's listed BC's. Before I read Bryan's book, I was using Nosler's published G1 BC for a 180 gr, 308, Accubond. After confirming all my inputs and still finding much more drop at 1000 yards than the ballistic software was predicting, I began lowering the BC until my observed drops matched the software predictions more closely. When I read his book, I was amazed to see his measured average G1 BC for this bullet was nearly identical to what I came up with by lowering Nosler's BC to match what I saw in the field. I would have saved a lot of ammo had I read the book first!
More importantly, he presents G1 and G7 BC's. Very few manufactures list a G7 BC for their bullets. The average G7 BC has the potential to produce more accurate drop predictions than many of the manufacturers' single G1 BC's because the drag profile of typical long range bullets is more closely modeled by the G7 model than the G1 model. I think Sierra is one exception to this because they list multiple velocity banded G1 BC's. Bryan covers all of this in detail in the book.
I know Bryan is a regular contributor to this forum and he could likely provide you with more details and answers to your questions as well.
