Hype matters a lot, sometimes there are fake just as Buggys, he is the clear example you can get a bounty you are not worth.
Luffy still isnt worth 4 billion berries. 3 billion is ok. And at this point, it aint about personal power, but military. Luffy still doesnt have as much allies as any other Yonko, reason he is still behind, his allies will need to do some feats of control and conquer for him to have a greater bounty imo.
Now Law might be around 2.5 billion at best, while Kidd 2.2. Reasons? Their feats, power and notoriety, after all they defeated Big Mom in a 2 vs 1, so they should at least be worth 2 billion. With their feats, more pirates should join them and increase their military power, but they didnt lol.