Couple things come to mind. Length does not always dictate an increase in BC. Think 6.5mm -130grn Berger VS 130grn CEB. The CEB is the same length as the 140 Berger with only a marginal increase in BC over the 130 Berger. .550 vs .560 respectively (IIRC) with an oal increase of ~.021".
The BC calculated by the manufacturer is most likely static in nature, fired from a single rifle at a single velocity at similar air density. As the testing becomes more dynamic, with changes in velocity & air density one will most likely see a (possibly dramatic) change in ballistic coefficient.
I'm not guru or SME on these things, I only know enough to get myself in trouble
My statements on BC are just what i've experienced personally.