The demo page doesn't seem to work that way. In fact, if you resize the window gradually, it won't load new images at all. You have to wait for a second after resizing for it to bother loading the new image.
From a practical standpoint, this is designed to address device-specific uses where the screen-width is less-likely to experience size-adjustments than a desktop browser, so you'll probably see the initial HTTP request for the small-resolution image, followed by the HTTP request for the appropriately-sized image for whatever device/screen you're using, and you're less-likely to see the same number of requests made while wildly resizing a demo page.