differences in the authentication scheme - could it be because of Kerberos?
When the auth_scheme is SQL, that means the user is using SQL authentication, which does not, and cannot use Kerboros.

When a user that is using Windows authentication connects, authentication will first try to use Kerberos.  If Kerberos fails for some reason, authentication will fall back to NTLM.  Probably the most common cause of Kerberos failure is that the correct SPNs are not present in Active Directory (see Register a Service Principal Name for Kerberos Connections.