Jeez I’ve been fiddling with this one issue off and on for almost a month and finally figured it out. Yes you are using the same image in the site background as you are in the slider background. But the difference between your demo and my site was you are using wide layout and I was using narrow. With the narrow layout, using the same image for site/slider background results in this:
http://gyazo.com/1c27ba440335ace052956924db121502
But in wide layout it looks like this:
http://gyazo.com/d808fcc227f4aa90990d02958ef92601
Clearly, I have much work to do still, but I feel a lot better about continuing, knowing my site is going in the direction I wish.