Say you see user2 listed in bold on user1's page. That means that user1 is also in user2's top 20 users. In my experience it is often an indicator of a good match (but not always). I should probably explain that on the site.
Instead of making it binary, you could use a gradient indicating the strength of the mutual correlation (like how HN colors downvoted comments).
The non-bold are dead accounts I think
It isn't due to a mere property of the user, as, for example, cushman is not bold as the #2 result for tptacek but is bold as the #2 result for icambron.
FYI, the GP said above that bold usernames are those for which symmetry holds (ie they're both in each other's top ten).