What 'resets' the transaction log
You are backing up the log to the same file repeatedly. Subsequent backups are being appended to that file, which is what you can see in your screen capture. Typically, we tend to backup to dated files:

BACKUP LOG 
    lagerdb 
TO DISK = N'C:\SQLBACKUP\lagerdb-rechts_201112061130.trn' 
WITH NAME = N'lagerdb logs'


Save yourself the trouble of hand cranking the necessary scripts and use Ola Hallengren's maintenance solution.