morga made sanji look like he was the head of germa working under luffy's command, the WG believed that hence the bounty increase.
the marines said water 7 was the first time they heard dragon's full name (maybe garp forced them to keep it a secret though), even in MF there were people who didn't know about that like smoker.luffu's increase was because he punched a celestial dragon, escaped empel dawn (a massive feat), assisting in the war, and invading MF again to start a new era. sanji only got his father's name and morgan's lie.