@Lykke: Play with the font hinting toggle in the bottom menu.
I seem to remember that we default to auto-hinting for... reasons. That may not be the case of the stock reader.
No hinting should be perfectly fine on high DPI eInk.
Native hinting may be preferable on low DPI eInk (or at tiny font sizes, provided they're hinted properly).
Regardless, we're using modern defaults (LIGHT render, i.e., no hinting on the x axis), so, differences are to be expected.
----
Possibly relevant:
https://github.com/koreader/koreader-base/pull/1171
(Because the refresh type may affect perceived contrast).