The widget displays different posts depending on post date.
If you have the widget in the sidebar and you view a post that is from the 1st of January 2013, the widget ONLY shows those posts up until 1st of January 2013. But if you view another post from the 7th of March, the widget displays the all the posts up until the 7th of January.
In other words, the widget does not display all the latest post all the time. It all depends on when the post that you're currently viewing was written. This is weird!