It is because when you scroll the page, the image is not aligned to the screen the same way.
The interlacing process is slow (for a computer, even if it looks fast) and it would be impracticable to redraw in realtime with the current technology.
Furthermore, I am not sure it would be possible to know exactly if it is displayed on an odd or even line.
However, when you click on the image, it will position the top always at the same place in the browser, so the same setting should probably work the same way from one image to another.
In advanced parameters, you have the possibility to switch the polarity of the lines.
You can also type X key to quickly swap the images, but it will not be saved.