Because Aokiji won't getting one, and he is near-equal to Akainu.
If Aokiji > Kizaru, then Kizaru having 3 billion bounty would make sense.
Kizaru was also shown as weaker than Shanks;
And struggled to defeat Marco, he needed Kairoseki against him;
Garp or Sengoku also having 3 billion bounty would make sense as they are old now, not in their prime, and Ryokugyu was wifi-diffed as well as Fujitora didn't look stronger than Ryokugyu.
So CG bounties might be accurate if Aokiji gets a bigger bounty than 3 billion.
If Aokiji > Kizaru, then Kizaru having 3 billion bounty would make sense.
Kizaru was also shown as weaker than Shanks;

And struggled to defeat Marco, he needed Kairoseki against him;

Garp or Sengoku also having 3 billion bounty would make sense as they are old now, not in their prime, and Ryokugyu was wifi-diffed as well as Fujitora didn't look stronger than Ryokugyu.
So CG bounties might be accurate if Aokiji gets a bigger bounty than 3 billion.