Because we are talking about 100 M increase - that is one hundred million. That is not a small amount at all. esp when there are other's in the crew that also got increases. Between Luffy (100), Zoro(200), Sanji (100) & Usopp (160) alone that's over 5 hundred and 60 million beris added to the SH crew combined bounty.
Maybe they had to spread it out:ninja:
to put it in a joking way - I'm guessing the Marines do have to put a limit somewhere or else they wouldn't be able to pay up for the bounties, backed by tenryubito or not. :ninja: