Coby received that CG bounty for babysitting some fodder civilians, while Law was pulling strings and Teach/Wang Zhi were going at it.
He is a hero, just like a firefighter is a hero. He did well protecting lives that day. But yea, because a hero title is held in great regard in the Navy, CG values him higher, cause taking him out is a decent hit to the Navy's image. It's a morale booster for pirates.
Every bounty has to be analyzed on its own and contextualized. Every...single...one.