Discrepancy between system catalog view and sp_change_users_login?
First, sp_change_users_login is deprecated feature and is discontinued in SQL Server 2014.  

Secondly, EXEC sp_change_users_login @Action = 'Report' is not a proper way to get orphan users list. As with 'Report' parameter, it will execute following query. According to which it will return users where issqluser = 1 (It must be a SQl Server Login), and no result if orphan user is NT User instead of SQL User.   

  select UserName = name, UserSID = sid,* from sysusers
            where issqluser = 1
            and   (sid is not null and sid &lt;&gt; 0x0)
            and   (len(sid) &lt;= 16)
            and   suser_sname(sid) is null
            order by name


According to BOL "sp_change_users_login cannot be used with a SQL Server login created from a Windows principal or with a user created by using CREATE USER WITHOUT LOGIN."

Best script to find orphan users against all databases on an instance is already provided by "Mark Storey-Smith"  

Find orphaned users