It's not just that the fill-factor on OLEDs is lower than LCD or other display technologies, the other issue is that LG's displays use a WRGB pixel structure so you often have fewer subpixels illuminated at once.
On an RGB display showing white, all subpixels are illuminated. On a WRGB display only 1/4 of the subpixels may be illuminated, making the screen-door problem even worse.
On an RGB display showing white, all subpixels are illuminated. On a WRGB display only 1/4 of the subpixels may be illuminated, making the screen-door problem even worse.