The older a post is when the (valuable) upvote occurs, the closer the curation reward ratio is at 25%. You can see this for example in this article (curation share is ~24%) by a user who pays bots for upvotes on articles that are ~6 days old to get back his invested capital after 1 day and doesn't have to wait a whole week. The downside of this approach is that most of the possible earnings are eaten away. It's almost not profitable to do so.