IMHO the author didn't knew how high the bounties would get later in the series. While it has been explained that the Shichibukai bounties are frozen and don't represent their current threat, when Crocodile's bounty of 80 million was first mentioned, the reaction was that of it being the real deal.
When Smoker was talking with the Strawhats about Robin or Crocodile bounties, he said that if the two joined forced they would literally endanger the world.
There's also Dorry and Broggy, it was said that their crew terrified the world, however their bounties was mere 100 million.
More recently, Mother Caramel stopped the execution of some of their crew members in Marineford, arguing that it would endanger Humanity.
