Payment Process Profile Document Grouping

Why is it that upon grouping of payment documents (invoices, debit memo, etc.), all Debit Memos are deducted on the invoice/s with the latest Due Date?

I have set the Payment Process Profile document grouping to "Due Date" under the Payment creation tab.

For example:
INV-001
type: Standard
amount: 100
due date: 20-FEB-2012

INV-002
type: Standard
amount: 250
due date: 25-FEB-2012

DM-001
type: Debit Memo
amount: 25
due date: 20-FEB-2012

DM-002
type: Debit Memo
amount: 50
due date: 25-FEB-2012 

Upon running the payment process request, the proposed payment created would be:
Payment1: 100 (for INV-001)
Payment2: 175 (for INV-002, DM-001 and DM-002)