Bounties aren't always reliable for powerscaling.
If you try to powerscale Mihawk off his bounty, for example, that'd work because his bounty is based mostly off strength and reputation
If you try to powerscale Snack off his bounty, you'd have a problem, because we see him get trashed by 100M Urouge, so we can't really assume it's based off only his strength, as it'd be contradictory.
The fact that Luffy beat him despite having a lower bounty proves it.
It's easier to assume his bounty is based off his reputation as an invincible man who never laid on his back and what not, along with being BM's son and the strongest crew member (besides BM her self obviously) than it is to assume it's purely due to his strength.