Oda cares about years and inflation
We see that some warlords did not do any feats but suddenly received an increase in their bounty. This shows that inflation has affected their bounty and increased it.
For example, Kaido did not have a bounty of 4.6 billion from 20 years ago until now, but now he has it.
When Shanks was 26 years old and was fighting legendary battles and even attacked CP and stole the most dangerous fruit in the world, he barely got 1 billion bounty, while now Robin has the same bounty, while he is not comparable to Shanks in any way.
And......