RMA Credit Memo Changing Old Already settled receipt/Customer Balances by Cash Discount amount

Hi,

We observe below case in our instance (12.0.6)

1) Created AR Receipt for Customer

2) Attached Invoice to above receipt and receipt got settled .

3) After few months RMA Credit memo created against above Already settled Transaction(Invoice) and that CM get auto apply to Transaction in current open period .

4) Old Transaction Line got reversed and also Cash Discount get reversed .

5) New line get applied with new Discount amount

6) Receipt get open for applied Credit Memo

Our Query

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1) Why discount get reversed ? because initially when customer made payment he made in time and gained Discount why that is to be reversed ?

2) Why Difference is coming in old settled transactions ?