Chris would be the one to answer that for sure, but without seeing the "plumbing" underlying the site, my guess would be that the difference is that being orphaned or unconnected is a fairly simple test which can be discovered through looking at fields in the database, whereas having a profile be unsourced would entail parsing the main part of each and every page. It's technically doable, but it would be extremely compute-intensive, and from other comments Chris has made, I gather that processing time is a fairly major constraint on the site.
In the short term, you can use this trick: go to the Unsourced Profiles Category and click on the "Next 200" link at the bottom of the page. That should get you a URL something like http://www.wikitree.com/index.php?title=Category:Unsourced_Profiles&from=Ackerman-537. Edit the URL in your address bar to replace the existing ID with "Mclean-1" (or whichever last name you're working on at the moment), so you end up with http://www.wikitree.com/index.php?title=Category:Unsourced_Profiles&from=Mclean-1, then hit Enter, and you'll see all the unsourced Mcleans. I see that the system is case-sensitive, and alphabetises lower-case letters after upper-case letters, so any time you're searching on "Mc" or "Mac" names, be sure to search for both lower-case and uper-case variants (Mclean-1 and McLean-1).
In the long term, I suggested some months back that there be "source" fields for the major life events (birth, christening, marriage, death, burial, and possibly a few others), and having more of a "training wheels" system to build a beginning biography from those fields, so people can see how to cite other sources that they find. That would, of course, use more disk space and processing time, so even if Chris thought it was a good idea, it might have to wait until one of us wins the lottery. But should that ever happen, then having a report showing which profiles lack sources for those major events should be pretty simple. (You could even get all fancy with it, and show reports as having sources entered for X% of the events for which there is a place and/or date.)