NDMP Differential Backup on NetApp

Hello,

i make backups from a netapp filer FAS2040 , NetApp Release 8.1.2 7 .

I make one full backup in 4 weeks, and one differential backup per day. The full backup has 6 TB on size.

The differential backup has on the day after full backup 20-30 GB, but is growing day by day.

On the end of month i have sometimes one terabytes of data in a differential backup for one day. This is not possible.

I use netbackup server 6.5.4. on solaris 10. I use ndmp over network (not direct write to tape).

I use NDMP also for sun/oracle open storage zfs box. Here is all ok.

Where can be the problem ?

Greatings

Christian Steinborn