Ref T11816. Since the dashboard got updated, Differential now interprets "Responsible Users: epriestley" to mean "epriestley, or any project or package epriestley is part of". You can query for just "epriestley" with "exact(epriestley)".
Give Calendar invites the same behavior: "epriestley" means "any event epriestley is invited to, or a project they are a member of is invited to". Individual invites can be queried with "exact(epriestley)".
This is a little bit copy-pastey but I want to wait for a third use case to clean it up since I think I'm going to have to do a bunch of generalization around "how does an individual PHID get turned into a bunch of PHIDs".