The issue is a result of our forum migration. When we moved from our old forum software (which was no longer supported by the vendor) to our new environment that uses vBulletin, we are able to import all the old posts however we were not able to maintain the links between posts and users for one of the user groups. (That group included Phil and Jonathan.)
Since the migration, which occurred in June 2008, all new posts by those users are linked to their account.
I've been investigating a way to fix these old posts and relink them to their users. I've had only limited success so far, but I'm still investigating.