Problem is that your answer isn't exactly simple.
Drag is proportional to the square of velocity, However... Way more factors are at play than just that.
The "steps" you see in listed G1 BC numbers are as much a description about the change in air flow around the bullet, as it is a simple description of "slipperyness".
All bullets fired have a natural yaw to them, and procession around the center of rotation. Those factors also play a role, as a sideways skidding bullet isn't very efficient; regardless of how sleek it may be.

More specifically to your specific bullet question: Your bullet looks nothing like the standard G1 projectile, and you are only given ONE G1 BC number, which doesn't exist. Should you really be surprised that the numbers don't jive? No.
So: how did Hornady come up with that number? Did they fudge it based on a mathematical calculation? Did they take an average of observed results? If so, over what set of distances?

For the overwhelming bulk of Non-hunting bullets (flat base, round noise types) G7 ratings are MUCH closer to reality, until trans-sonic. At that point the flow of air around the bullet shows the G1 system to work very well, regardless of bullet shape.