Whitebeard was the strongest man in the world. He was the captain of the Whitebeard Pirates, directly controlling an enormous pirate fleet of 1,600 pirates divided into 16 divisions, each one being led by an incredibly powerful division commander.
Unlike Roger and Rocks, Whitebeard was not a D. and he wasn’t trying to find taboo subjects (Void Century, Ancient Weapons, etc.).
He was content with pirating with his group he used to call Family and he wouldn’t start a war UNLESS you mess with his crew (despite having a devil fruit that can destroy the world and being the world’s strongest man).
Still Whitebeard gets 5 billion bounty!
While Gol D. Roger was actively seeking topics directly prohibited by or against the World Government’s ideals. He held the title of Pirate King and owned the legendary treasure known as “One Piece”.
The Roger Pirates were the only crew to ever reach Laugh Tale, the end of the New World in the Grand Line. On the final island, they found a treasure left behind 800 years ago by Joy Boy and learned the truth behind the Void Century, Ancient Weapons, and the meaning of the D.
For these reasons, Roger obtained 5.5 billion bounty. Only higher by half billion than Whitebeard’s.