BACKUP ... FORMAT '%U' Default?

In Oracle documentation I read that when issuing the BACKUP command without FORMAT and by issuing the backup command with FORMAT '%U' the filenames should be the same. That's why '%U' is considered default. See below link (Scroll down to FORMAT clause):

http://docs.oracle.com/cd/E11882_01/backup.112/e10643/rcmsynta007.htm#RCMRF107

So I test this in my environment.

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First I issue:

backup as copy datafile 1;

and the filename I get is O1_MF_SYSTEM_881NQOC2_.DBF

Not the '%U' format.

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Then I issue:

backup as copy datafile 1 format 'C:\app\fast_recovery_area\dhman\DATAFILE\%U';