I think your document must be having enough space in the window to display its contents. That means there is no need to scroll down to see any more part of the document. In that case, document height would be equal to the window height.
Basically, $(window).height() give you the maximum height inside of the browser window (viewport), and$(document).height() gives you the height of the document inside of the browser. Most of the time, they will be exactly the same, even with scrollbars.