You are absolutely right that a good goat is 9" and a monster is 9 1/2+. There aren't many guys that can distinguish fraction of inches between average and B&C billies. Similar to other horned species, mass is super important and is a high percentage of the score. Horn length is actually over rated.
You’ll also notice in the B&C books that giant billies tend to come out of certain small zones. Some regions in Alaska are known to produce quite a few B&C entries.
Another thing about goats is they are similar to antelope and can reach B&C in only a few years compared to antlered species like elk. This is generally 4 1/2+ years in goats vs 7 1/2+ years for elk. Obviously there are exceptions. I've seen quite a few 9" billies at 3 1/2 years of age. Once goats reach 9" length there is generally super small horn ring growth in years after this. Billie's generally takes around 4 to 5 years to totally mature and reach maximum body size.