Because the scenario tells you: “The difference in the balance is due to an invoice which is under dispute due to faulty goods which were returned on 29 June 20X6.”
So that Carnation is paid 50 after the year end doesn’t confirm that only 50 was owed, because Carnation’s statement says that it is owed 70.
Please remember to use the Ask the AA Tutor forum if you want to be assured of some response – you will see that I answer posts every day.