http://bl.ocks.org/jfirebaugh/raw/fd2d26ef85831783f149/

Look at the x positions logged to console on mousemove event. They are off by a significant amount. Depending on your browser width, the mousemove events might not even get dispatched. If not, widen the window.

Trigger seems to be:

        body {
            position: relative;
            max-width: 500px;
            margin: 0 auto;
        }