There also might be an impact from how older vs newer fic is read. Due to plain laziness I am less likely to comment on fic I downloaded and read on my tablet than fic that I read in parts where I stay on AO3 website or the journal where a comment field is right there. Old fic is usually finished, so unless it's really short I usually download the whole. Also if go to a new story that is linked from a journal, even if I download, I often still see the announcement on my flist, and I can comment on that entry rather than on the archive.