X10 vs X20 Studio Comparison

I don't see where the X20 is "slightly better" except for that it shows more structure and reflection points on the background curtain. Furthermore I wouldn't put ​too​ much into these comparison shots, because they are JPG with seemingly different processing and especially because the lighting and shooting angle was ​not​ fully controlled (you can see it in the reflections on the glas, even including some additional shadows on a reflected white door).

Nevertheless, judging by that one comparison the X20's JPG engine used both stronger sharpening and stronger noise-reduction (in that comparison) and thus loses in detail versus the X10. This would explain what you called "unexplained black pixels", the JPG engine of the E-M5 does the same.

Fujifilm even explains how their "new processor" increases resolution and reduces noise, which is just what I just perceived by looking at these JPGs: stronger filtering.

http://www.fujifilm.com/products/digital_cameras/x/fujifilm_x20/features/page_02.html

Here an emphasized demonstration at ISO 1600:

X10 (upper) vs. X20 (lower)