Thanks for your reply. I think I figured it out:
When I did the XML imports it took a few tries to get it right for several of my back issues, but eventually they all appeared correctly in the issue browse. My issue tags looked like <issue published="true" current="false" identification="title"> for each issue, so I did have published set to true. What I just noticed is that although the titles matched up in the unassigned and archives submissions screens, the article ID's were mutually exclusive--in other words, I think my unassigned submissions are actually the leftover detritus of my failed upload attempts. I remember my uploads failing for various reasons, including spaces in the file names, missing </tags>, missing quotation marks, etc; on some of the attempts the files must have made it through before the upload failed. Now I guess my only recourse is to archive and delete each bogus submission one at a time....