This means that the user record was created by either a server-side Add User Properties API call or by one of our Integrations (ex: Salesforce). Once the user is identified client-side with our identify API, the two records will be merged correctly.
If you see a message that events are hidden, this means the user has only events sent via our server-side Track API. Please check your Hidden Events category in the Events tab to see a range of events that the user may have done.