You can find some more information here, but the best explaination seems to be this one.

If you vote immediately after a post is made, then 99.94% of the curation reward will go to the author.

So even if the post made over 300$, if you voted in the first seconds or minutes most of the reward goes to the author.

The ideal situation for maximum rewards is voting on a post that is at least 30 minutes old, has only 1 vote from the content creator and ends up making $50,000.

You have to find a middleway to be one of the first upvotes but don't vote too early so more reward goes to the curators.