Exam CFA® Level 2 topic 1 question 100 discussion

Stanley Bostwick, CFA, is a business services industry analyst with Mortonworld Financial. Currently, his attention is focused on the 2008 financial statements of
Global Oilfield Supply, particularly the footnote disclosures related to the company's employee benefit plans. Bostwick would like to adjust the financial statements to reflect the actual economic status of the pension plans and analyze the effect on the reported results of changes in assumptions the company used to estimate the projected benefit obligation (PBO) and net pension cost. But first, Bostwick must familiarize himself with the differences in the accounting for defined contribution and defined benefit pension plans.
Global Oilfield's financial statements are prepared in accordance with International Financial Reporting Standards (IFRS). Excerpts from the company's annual report are shown in the following exhibits.


What was the most likely cause of the actuarial gain reported in the reconciliation of the projected benefit obligation for the year ended 2008?