WordPress.com introduced a new panel to your stats page, "Top Authors". Here's how it works: all the posts that were visited during the day are counted up, divided by who wrote them. The author with the most visits across all his or her posts of the day, gets the top spot."
To see each author's posts, you can click "plus icon" to expand and reveal just what stories earned them their points of the day. Because that was a popular post, it earned her the top spot.
"Because we liked the grouping feature so much, we also added it to your referrers box, which now bundles up links from the same domain," WordPress.com blog said.
The new panel is available for all blogs on WordPress.com. It'll appear automatically if you've two or more authors with traffic on their posts.