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.
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.
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).
Good point.