The recent revisions show media revisions which are older than the page revisions. Could it be that $conf['recent_days'] is either not used by the media revisions or used in a different way?
See an example here:
http://www.dokuwiki.org/?do=recent&id=start&show_changes=both&first%5B120%5D=less+recent+%3E%3E
There you can see changes to media files from 2010.