Thats probably b/c during the first 30 minutes of a post the author/curator reward it different. If you vote as soon as the post comes out then the author gets all of your curation rewards but if you wait until 30 minutes after the post you would get the whole portion based on the weight of your vote. And for example at 15 minutes you would only get half of earned curation. That is my quess to why its 80/20 . If the 30 minute rule didnt apply it would be 75/25 if i understand correctly.

Edit: I didnt see your additional information stating what i wrote above. :)