Pen-F : the best Olympus ever for image quality ?

Yeah, there is an explanation. First, it was pointed out somewhere that dpreview used an inferior lens on the E-M1 II - and the camera might not have been ready for release. Second, it depends on what images you're comparing.

Other comparisons on sites like SLRclub and Imaging Resource show the E-M1 II having better detail and control of noise in JPEG than the PEN-F. I haven't seen enough RAW comparisons, but still, regardless of lens there are some worthwhile things to note from the dpreview comparison tool:

High ISO RAW - PEN-F vs. E-M1 II vs. GX8: overall, to my eyes, the PEN-F wins for noise, detail preservation, and color. Even if the lens is responsible for the latter two points, the PEN-F's shadow noise advantage (especially in the blue channel) stands out. The GX8 comes in second in this comparison.

Hopefully, there is nothing going on behind the scenes (like exposure differences) that would falsify this comparison. It's a lot harder to compare noise through a whole range of grays and colors on other sites.

JPEG is hard to compare, not only because of lenses, but because the testers only use the default settings. Still, we get a sense of noise control at high ISOs - and it depends on what you compare. Beyond ISO1600, I think Olympus JPEGs show less noise reduction mushiness than the GX8 and other new Panasonic cameras. (This matches my experience comparing my own GX85 and E-M5 II.) Even in dpr's comparison tool, the E-M1 II seems to pull ahead here. Still, in some places, I see better detail from the GX8 (ISO6400) - again, matching my experience with the GX85 and E-M5 II.

Personally, I like what I see from the PEN-F most. That's overall - the other 20MP cameras sometimes come out ahead. Between my own two 16MP cameras, the GX85 is a bit better overall, but not always - and that matters to me (e.g., the GX85 has a tendency to posterize JPEGs and turn skies to purple).