Can you explain this a bit more, I tried googling but I can't quite understand what you mean here by pixel fill factor and how it would differ between the resolutions?
The gaps between the pixels tend to be smaller the higher the resolution - so even if you drive the display at a lower resolution, it can look better the same lower res display that has larger spacing between each subpixel.
Interesting, does this give you a noticeable benefit for text, or does this mostly apply to images or video?