In order to understand the breaking down of curation rewards, first you have to understand that of the post value, authors obtain 75% while the other 25% is split between the author / curator. For this post, lets focus on the 25% aka the amount of rewards possible for curators to obtain.
To make this simple, let's start from the back and move towards the beginning. When a post has been live for 30 minutes and a curator decides to upvote it, the ENTIRE portion of that 25% goes to the curator PERIOD. If any curation rewards are gained on an upvote that has been made past the 30 minute mark of post publication, those rewards go straight to the curator.
The next "reward bracket" is what I'll call it, is the post-27 minute curator reward. In this bracket, upvotes made by curators between 27 minutes and 30 minutes past the time the post has been made live, will be split 90/10. Of the available 25% up for grabs to authors and curators, 90% goes to the curator and 10% goes to the author.
Following that will be the time period between 15 minutes and 27 minutes of when a post is created. When a curator decides to upvote a post during this bracket they are able to receive 50% of the reward while the other 50% goes to the author. AGAIN, this is 50% of the AVAILABLE 25% curator post rewards that you can possibly obtain.
The smallest bracket where curators are able to obtain rewards is the period between 3 minutes and 15 minutes after post publication. In this bracket when a curator upvotes, they are able to work towards 10% of the total 25% curator rewards while the author receives 90% of it.
Finally, there is the period where I like to call the author bracket. During this time or right when an author posts to 3 minutes after the post has been up, the curator receives no possible curation reward. In this bracket, the entirety of the upvote in the available 25% curation reward is given to the author and the author only.
Why does SteemIt do this you ask?!