A ballistic coefficient is just a mathematical model of a given bullet compared to a theoretical bullet of similar shape. This theoretical bullet is called a G1, G2...G7, etc. At different speeds the model either matches closer to the actual bullet, or further away. The variation is compensated for by changing the coefficient. The closer/more perfectly the model matches the actual bullet, the less variation there is in BC over a wide range of velocities. This is seen by comparing the G1 BC of a VLD-type bullet over a wide range of velocities, with the G7 BC figures. Form factor is a bit better way of modeling a bullet's aerodynamic properties compared with other bullets, but it still relies on modeling the bullet based on a theoretical bullet shape.
