Prior period adjustment

Hi, I have this question from Dec07. I can’t understand why the prior period adjustment is made at 1 Oct 2005 instead of 2006. Please help. Many thanks.

Statement of changes in equity
Prior period adjustment (credit required to restate retained earnings at 1 October 2005)
(cumulative carrying amount at 2005 of 300 + 165) 465

Question:
Emerald has had a policy of writing off development expenditure to the income statement as it was incurred. In preparing its financial statements for the year ended 30 September 2007 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: $’000
Year ended 30 September 2004 300
Year ended 30 September 2005 240
Year ended 30 September 2006 800
Year ended 30 September 2007 400
All capitalised development expenditure is deemed to have a four year life. Assume amortisation commences at the beginning of the accounting period following capitalisation. Emerald had no development expenditure before that for the year ended 30 September 2004.
Required:
Treating the above as the correction of an error in applying an accounting policy, calculate the amounts which should appear in the income statement and balance sheet (including comparative figures), and statement of changes in equity of Emerald in respect of the development expenditure for the year ended 30 September