Interesting link here on determining ballistic coefficient of bullets written by Bryan Litz, Ballistician for Berger.

http://www.appliedballisticsllc.com/index_files/AccurateSpecificationsofBallisticCoefficients.pdf

I didn't realize prior to reading this how complicated determining BC of a bullet was. Seems like bullet makers have quite the task. And of course, then someone is unhappy when they shoot under "other than tested" conditions and get different numbers. I'm not defending bullet makers, but I believe, like velocities, they just report what they saw in the test.