What the World Government thinks, and what they know is two different stories... They "Think" Buggy is this legendary Pirate, strictly based off of hearsay, and the fact that he somehow got Crocodile and Mihawk to become his subordinates. He has not Worldwide strength accomplishments. They are going off his rise to fame, which they have no clue is just based off of dumb lucking his way to the top. That's why you don't see Buggy with a Bounty that Rivals Shanks, even though he "supposedly Rivals him.", because Buggy's bounty has nothing to do with individualism. It's all about the strength of his crew, which happens to consist of Mihawk and Crocodile.
On the other hand, they KNOW Mihawk's strength and accomplishments. His bounty is placed above Buggy's, for that reason. One's mythos is based off rumors, and dumb luck, while the others is based off of cold hard facts.
Buggy just gets the benefit of the doubt because of who his subordinates are, so he has a decently high bounty, but even though he "supposedly Rivals Shanks", he doesn't get the same bounty treatment, because if the WG truly believed he rivals Shanks in strength, then his bounty would be MUCH higher.
And yall want to keep using this same argument, "Buggy Rivals Shanks so, bounty" then give everyone a good reason as to why Buggy, who rivals Shanks in strength, and supposedly has the strongest man in the multiverse as a subordinate, doesn't share an equally high bounty as Shanks.
If Mihawk is stronger than Shanks, and Buggy is supposed to be as strong as Shanks, then it makes no damn sense that Buggy's bounty is so much lower than Shanks.
The only possible explanation, is that the World Government doesn't believe for a split second, that Buggy is comparable to Shanks in strength.