Why do People Scroll Up?
When the user scrolls down, the header jumps out of the way. This gives the reader a clear view of the content. If you begin to scroll up the page, the header comes back into position and makes the assumption that we're looking for navigation-type links.
Here's a demo detailing the behavior:http://codepen.io/assets/embed/ei.js"> </script>
I think this is a nice touch - a little something extra to enhance my experience.
Why do you scroll up?