Mihawk's bounty being below 1.5 Billion is possible since 1.5 Billion enters into Emperor threat level (Luffy got dubbed Fifth emperor with 1.5 Billion and we haven’t seen any pirates other than the emperors themselves getting 1.5+ billion).
It wouldn’t matter how strong Mihawk is, he is not an Emperor nor has invaded an emperor territory, so below 1.5 Billion is not out of question.
It's not anything to fret over.
Edit: Forgot that Law and Kid got 3 billion now. But they beat an Yonko, so I think it is still okay.