@Chaves It’s as a lot of people are saying, it’s to suppress his importance as much as possible. The 3 of them defeated 2 emperors so you can’t have their bounties too low so the best thing to do is to have all 3 of them with the same bounty (so Luffy doesn’t stick out in any way.
Another thing, the WG controls the bounties not the assignment of Yonko titles, which seems to be done by the media and popular consensus (Morgan’s is the king maker). I’m sure they were just as surprised as Luffy being named emperor as the rest of the world. It blatantly goes against what they were trying to do.
I believe that’s why Oda did the bounties like that, to show that the government tried and failed to cover up Luffys importance; or else why have the 5 elders try to stop the press.
(Another smaller reason for the bounties being that way is the 9bil>8.99bil and also Luffy seems like he’s gonna get 5.6bil as his last bounty).