[keycloak-user] is resource owner username or userid

Hi, I use keycloak 2.4.0.Final with spring boot adapter, and authz-client -authz-admin. When i set owner , i set (getAccessToken().getPreferredUsername()) (my user name, admin ) But when i try to get resource owner ($evaluation.getPermission().getResource().getOwner()), it returns userid not username. is it wrong ? or do you these purposely ? is username unique ? why does we use username ? thank you for helping