RMan Backup Piece count varies between backups

I am using Rman nightly database backup on 11.1.0.7 on Linux64.

I have noticed the backup piece count varies between nights from 52 one night & other nights 58.

As below:

52

piece handle=/backup/rman/auprac/AUPRAC_datafile_08mgnc68_51_1_10248_51.bak tag=AUPRAC_DATAFILE_DAILY comment=NONE

channel ch1: starting piece 52 at 06-JUL-11

channel ch1: finished piece 52 at 06-JUL-11

piece handle=/backup/rman/auprac/AUPRAC_datafile_08mgnc68_52_1_10248_52.bak tag=AUPRAC_DATAFILE_DAILY comment=NONE

channel ch1: backup set complete, elapsed time: 03:46:53

channel ch1: starting compressed full datafile backup set

channel ch1: specifying datafile(s) in backup set

including standby control file in backup set

including current SPFILE in backup set