The answer is that Oda doesn't care about bounties as much as you do. He probably did this just for the jokes plus some wordplay.
Sanji having the third highest bounty is the uncommon thing; doesn't stop him from remaining the third strongest and a member of the monster trio whether someone bar Luffy and Zoro has a higher bounty than his or not. Next time Jinbe will (again) be paired with an opponent weaker than Sanji's and this whole "number 4" bait will again disappear.
Sanji having the third highest bounty is the uncommon thing; doesn't stop him from remaining the third strongest and a member of the monster trio whether someone bar Luffy and Zoro has a higher bounty than his or not. Next time Jinbe will (again) be paired with an opponent weaker than Sanji's and this whole "number 4" bait will again disappear.