You don't need to know everything, there are things happened in the past and we learned that long after.
- During Fishman Island, Jinbe didn't mention BB defeating WB pirates remnants, he only said BB conquered WB's territories, but he mentioned about Aokiji vs Red Dog, we learned that Payback War later during Zou. Even though Robin said because of Payback War BB become Yonko, then why Jinbe didn't mention that? Oda is hiding things for the plot.
- No one mentioned BB defeating Ochoku as well, we only learned that during Amazon Lily.
There could be many things that BB did during that timeline we don't know yet.
How did Shanks become Yonko 6 years ago? What did he do? There are a lot questions.
Imo i think they raised it because of kuzan officially joining. Kazan's bounty likely being higher than blackbeard's previous 2 billion berry bounty they had to raise it to be more precise to his true strength.
Basically I think it kinda works similar to how defeating someone stronger than you effect your bounty. Like if law beat kizaru or had him join as a subordinate the world government would likely assume in both situations he's stronger
Then why Buggy has less than Mihawk's? Lmao.
Aokiji joined BB during Dressrosa,
Gorosei already knew it long before when BB had 2.2 billion.
We know right now crews are irrelevant to the bounty due to Buggy having less than Mihawk, Buggy only got 3.1 billion for his strength that superior to Mihawk. (Thats what Marines think).
Same as
Kid having Killer in his crew, but equal bounty to Law's bounty. If you don't believe Law>Kid, crews don't boost their bounty.
Luffy also has three people who have bigger bounty than 1 billion, and he only has 3 billion for himself.
Yonko subordinates don't boost the Yonko bounty, Yonko boosts the underling bounties.