Because it is correlated. Bounty= strength + threat level to the WG. And thus more strength = more bounty. Being a bigger threat to the WG also = more bounty. Even killing innocent ppl increases your bounty.
jinbe has a history of allying himself with yonko crews and not joining them fully. he did it with whitebeard and big mom. there is good reason to think that he was an ally of the strawhats and not a complete member. i assume that's the mistake the beast pirates did.
That is what I am saying. Pirates who has never seen a pirate crew members fight has nothing to go on but reputation and bounty to rank the other crew's members. Experienced or not‚ you can't tell who is stronger only by images