Whitebeard was the strongest man in the world.
He 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 calls Family and won’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 he 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.
Still Roger has 5.5 billion bounty. Only higher by half billion.