There actually is! As I've already mentioned in my earlier post, Oda probably feard that he would ran out of future bounty hype if he gave Luffy the bounty that he deserves. Say whatever you want, but people still get hyped about the new bounties. Oda can't increase Luffy's bounty forever, he can't go to 6,7,8bill etc. So he had to limit his bounty for future arcs like Elbaf for example–not the best choice, but it is what it is.
I'm not talking about Luffy ffs ...
I'm talking about Law and Kidd.
Luffy getting 3 billion is fine, but Law and Kidd both getting 3 billion, too, is just complete nonsense.
Nothing in the story justifies that. Heck, not even anything the marines know from a story standpoint (inner-manga logic) justifies that and Law is one of my favorite characters btw.
To then somehow make Luffy an emperor and the other two not, while they actually gave all of them the same bounty (mind you: bounty = threat level according to the lore) is just another questionable decision.
But who cares, overlord Buggy is Yonkou (this is a running gag I really did btw lol).