Why does the Install Base Mass Edit screen only return 600 IBs related to an account number?

My user is trying to terminate a few thousand IB records before next week's billing run.  He goes to the Install Base Mass Edit menu choice from the "Install Base Agent User" responsibility, and creates a new batch for the purpose of terminating IBs.  After entering a batch name, he clicks 'Add' to choose the IBs to add into this batch.

The search screen now allows him to search for IBs by a number of different criteria.  He enters an account number, and the system returns 300 IB records, all of which he selects.  Then he clicks 'Next 300' and after a while, the system returns with the next 300 records, which he selects as well.  However, at this point the system does not present him with the choice of finding the next 300.  He is limited to 600 records.  There are well over 600 IB records owned by this account.