I don't think it would matter much, this is just Buggy's handcrafted bounty system according to his personal criteria.
It appears that rather than making bounty posters with individual rewards, he made what appear to be "bounty tiers", so he either put Garp in the same tier as other VCs or he made him as worthy as an admiral.
Regardless, if marines had received proper bounties, though, Garp still wouldn't be worth as much as Roger. He may have been as strong as him in his prime, but he's old like Rayleigh now, and he does neither own an army like a Yonkou does nor has the authority of an admiral, even if he's respected like a hero (which in turn would probably raise his bounty a bit since that would make him dying a bigger blow to the marine's morale).