These are new bounties issued after Mihawk joined the Crossguild, this isn't the Bounty Mihawk had when he rode solo.
The Marines clearly mention Mihawk's association with Crocodile and Buggy (who is named as a mastermind of the Impel Down escape, member of the Pirate King's crew and close acquaintance of the Emperor Red Hair), and the Guild's Marine hunting business getting backing as the reason for the groups threat and their new bounties.
TLDR;
Mihawk's bounty is not solely due to his strength.
Please learn to read and stop spreading bullshit.
No shit sherlock
Bounty = WG overall threat level
Literally nobody's bounty is ONLY Strength, the point is how many factors contribute to Mihawk bounty , that's only 2
- being a yonko subordinate , doesn't mean anything as Mihawk ISN'T EVEN CONSIDERED THE SECOND IN COMMAND so you could scale this to a 300 Million berry contribution , just look at Nami,Usop, Franky Increase
- Strength , so pretty much 3.3B IS STRENGTH ONLY
Now let's look at Shanks
- ex Roger Crew tax
- Leading the STRONGEST CREW IN THE VERSE
- Territories
- Fleets
- Yonko title
all of this is pretty much worth 2.5B -3B MINIMUM (just look at the reasons why Buggy got 3B+)
- Strength 4-3= 1B