Backup Exec always deletes expired disk backup datasets, even when there is free space available

My question is why Backup Exec always deletes expired disk backup datasets, even when it's not necessary because there is free space available on the same disk? I wanted to have as many copies as possible and this behavior prevents it. I am using Backup Exec 2012 version 14.0 Rev.1798 (64-bit) Thanks for your help