Please read this article from steemitblog.

In order to eliminate this advantage, the portion of curation rewards that is not given to the curators due to the early voting penalty will be returned to the rewards pool instead of being awarded to the author, thereby increasing the overall percentage of rewards paid to curators. This will better serve the original mission of the curation rewards budget: to ensure that the Steem blockchain distributes rewards to the most valuable content.

Only the part of curation that used to go back to the author goes to the reward pool. And it s not everything before 15th minutes, it increases linearly.