By default this is the behaviour you will see doing NDMP backups and the devices are not attached directly to the filer. NetWorker sends the data to the NetWorker server which then works out which devices it can use for the backup and then passes the data to the relevant storage node.
You can however configure the data to be sent directly to the Storage node you want to by adding flags to the backup command in the client definition.
From the Administration Guide I believe you have to add -M and -P to do this.
you will also have to add the NDMP client name into the Storage node servers file otherwise the backup will fail.
Please see NetWorker Admin Guide NetWorker Server Support for NDMP section for more details.