2.2B was simply way too low for a Yonko bounty, even a “new” Yonko (BB had already been a Yonko for like a year atp), and Oda realized that. Imo Oda should’ve had the thing BB was talking about “seizing before the Marines can take it” be something more grandiose than the fucking Mero Mero no Mi (which he didn’t even get anyways, and which also happened right when his 2nd bounty was revealed), but whatever.
Guys like Kid and Law got 3B bounties, and there’s a good chance that Ben Beckman imo will have an above 2.2B bounty, so BB’s 2.2B bounty is clearly just a result of Oda not knowing how high he would end up making the other Yonko’s bounties imo. I guess Brannew being on an acid trip when he give BB that first bounty would be the most valid in-verse explanation