In One Piece bounty doesn't necessarily equal strength, it's a perceived threat rating. Strength is a factor usually, but it's not the only one. From this, we can see that numbers aren't everything. Being able to lift more weights than someone doesn't mean you would beat them in a fight. Having more money than someone doesn't mean that you're better or more successful than they are. Having gone to school for more years than someone doesn't make you more knowledgeable. Just like bounty can be influenced by strength, so can the factors in the examples I mentioned. However, that's not always the case and there's more than one variable to it. Maybe you can lift more weights than someone, but you're slow which would put you at a disadvantage. Maybe you have more money, but your ultimate goal is something immaterial you haven't yet obtained (friendship, approval, love, etc.). Maybe you went to school longer but didn't retain anything you learned. And maybe your bounty is based on your connections or underworld business influence and not your combat ability.