Even though the Advanced Site Administrator group is often regarded as the "Super Admin Group",  these permissions are not automatically assigned to administrator groups and must be added manually. This particular functionality is set forth by the "Custom Permissions" category when editing the administrator group's permissions.
  1. Navigate to Setup > Groups.
  2. Search for the administrative group that you want to adjust the permissions for.
  3. Select the Edit Permissions link next to the group.
  4. From the drop down menu, select Custom Permissions and select "Choose"
  5. Mark the radio button for Custom Reports and save your changes.