The G1 ballistic coefficient curves don't match modern bullet shape drag functions very well, so one way to fix that is to adjust it at different speeds to get a better curve fit. For example, Sierra provides match bullet BC's this way. For example, the BC (G1 is the SAAMI standard) for the 175 grain MatchKing is given as:
.505 @ 2800 fps and above
.496 between 2800 and 1800 fps
.485 @ 1800 fps and below
It was entry of that data I wondered about. If the program accepts G7 BC's, though, I don't really need it. G7 matches most boattail match bullets well enough that one number is good enough.