t-log backup history

SELECT  
   * 
FROM   msdb.dbo.backupmediafamily  
   INNER JOIN msdb.dbo.backupset ON msdb.dbo.backupmediafamily.media_set_id = msdb.dbo.backupset.media_set_id 
WHERE  (CONVERT(datetime, msdb.dbo.backupset.backup_start_date, 102) >= GETDATE() - 7)  
		AND msdb..backupset.type = 'L' 
		AND database_name = 'MyDatabase'
ORDER BY  
   msdb.dbo.backupset.database_name, 
   msdb.dbo.backupset.backup_finish_date

hi all - i am using the above script to analyze my log backups for a particular database. this script is showing a bunch of records where thephysical_device_name is null and the user_name as NT AUTHORITY\SYSTEM. any idea what this means? thanks for the help.
also, can i find out what application did the backups - sql agent, third party tool etc?