Dione Reduces Vertical Resolution when not upscaling

Just went to see how long it would take to deinterlace a movie with no upscaling with Dione, Result was going to take over 9 hours so I cancelled it. I did however set no resize and the resultant file was only 640x480 down from the original 720x480 resolution. Not sure why the vertical resolution should have changed or why it should take this long with no upscaling involved? Just tried scaling to 768x576 and that was going to take 20 minutes less time!!