Chester Co – Study Hub

Chester Co receives monthly statements from its main suppliers and performs regular supplier statement reconciliations. There were inconsistencies noted in respect of the following at 31 May 20X5:
Poole Co
Supplier balance per listing (72,168)
Balance per supplier statement 84,235

Which of the following would be a valid explanation for the difference in respect of Poole Co?
An invoice for $156,403 has been paid twice
An invoice for $156,403 has been posted as a debit note
An invoice for $156,403 has been received and processed prior to receipt of the goods
A.1 only
B.1 and 2 only
C.2 and 3 only
D.1, 2 and 3

I chose B, however, the correct answer is A. I thought a debit note will also debit the account.
Can you please explain this question?

Iniss.