Prior period adjustment

Hi Mike, hope you could help me on this:

Emerald has had a policy of writing off development expenditure to profit or loss as it was incurred. In preparing its financial statements for the year ended 30 September 20X7 it has become aware that, under IFRS rules, qualifying development expenditure should be treated as an intangible asset. Below is the qualifying development expenditure for Emerald:
Year ended 30 September 20X4: 300,000
Year ended 30 September 20X5: 240,000
Year ended 30 September 20X6: 800,000
Year ended 30 September 20X7: 400,000

All capitalized development expenditure is deemed to have a four year life. Assume amortization commences at the beginning of the accounting period following capitalization. Emerald had no development expenditure before year ended 30 September 20X4.

Treating the above as the correction of an error in applying an accounting policy, calculate the amounts which should appear in the SOPL, SOFP, and statement of changes in equity of emerald in respect of the development expenditure for the year ended 30 September 20X7.

Hope the workings below will be clear to you (i don’t know how else to present it)

Answer:
20X4:
Expenditure 300,000

20X5:
Expenditure 240,000
Amortization 75,000
Carrying amount 165,000

Balance 20X5:
Expenditure 540,000
Amortization 75,000
Carrying amount 465,000

20X6:
Expenditure 800,000
Amortization 135,000
Carrying amount 665,000

Balance 20X6:
Expenditure 1,340,000
Amortization 310,000
Carrying amount 1,130,000

20X7:
Expenditure 400,000
Amortization 335,000
Carrying amount 65,000

Balance 20X7:
Expenditure 1,740,000
Amortization 545,000
Carrying amount 1,195,000

Statement of financial position
Development expenditure:
1,195,000

Statement of profit or loss:
Amortization:
335,000

Statement of changes in equity
Prior period adjustment
Added to retained earnings at 01.10.20X5 = 465,000

I got everything right except for the prior period adjustment figure which I got it wrong. Why is the carrying amount at year ended 20X5 used as the prior period adjustment instead of the carrying amount balance at year ended 20X6 (1,130,000)?