Credit Memo: Created as transaction vs Created against Transaction Lines

 We are on 11.5.10.2. We establish invoices by importing them from other system. i.e. transaction source of type 'Imported'. I create two different credit memos. One by using manual transaction source from transactions window. and apply it to an invoice.

Another credit memo I create by going to Invoice line and from actions menu create Credit. Such credit memo uses the same imported batch souse as that of invoice.

Now, I can apply/unapply credit memo creted from first method while as I can not unappy credit memo creted with the second method. Why? Can anybody explain me the reason, please?