I'm not an archivist, but I have friends who are, and we did a MA in Museum Studies together (a related field). I think they are looking for somebody who can research, sort systematically and work within classifcation conventions etc; basically work within exisiting ways of sorting documents within archivies , and develop new ways of working with the archive. Perhaps a PhD student is supposed to be good at this or something? Maybe PhD students are used to researching, sorting into order and using critical skills to make sense of info.
Hope this of some help!