@hfpop I've seen threads on the kobo forum about differences in fonts rendering on kindle and kobo (which is more or less the same issue). These things can be corrected by changing one value (panose) in the font with fontforge (but just now I'm too lazy to search it back) ; If I'm not wrong, GERGE is the person you should seek for advice.
There is also some tips at the end of the wiki page for using fontforge :
https://wiki.mobileread.com/wiki/Kobo_eReader
and IIRC there is a script to "koboify fonts" : the result seems to make them more the same when you use them on nickel and on koreader.
Whatever the result, these are some funny things to try :-)