WG gives bounties based on overall threat level to the WG, which factors in the person’s strength and crimes they’ve committed against the WG in the past. CG gives bounties based on the threat Marines pose to CG, which includes their strength and actions taken against CG in the past. Garp beat up countless pirates in the past, but none of them were a part of CG, so CG doesn’t care about that. Garp’s bounty is based only on his strength in his old age