Luffy always gets a higher bounty than who he defeats
Arlong, Crocodile, Gecko Moria, Doflamingo, Katakuri
so people thought Luffy was gonna get a higher bounty than Kaido, 4.6 b, after all he defeated Kaido
but this time the WG considered the defeat of Kaido and Big Mom an equal effort by the 3 (Luffy, Law and Kid)
so he added Big Mom's and Kaido's bounty, and divided it by 3