FSA: pension: actual asset return

text book: in page 243: q11: actual asset return =expected return + actuarial gain in go to page 242, under obligation, actuarial gain / loss is (925), while under plan assets, actuarial gain =784 which kind of actuarial gain in asset could be? why actual plan asset return = expected return + actuarial gain? and why actuarial gain/loss under obligation is not the same as under plan asset ? Thanks.

plan assets dont relate to actuarial gains or losses because the actuarial stuff has to do with future obligations (PBO) while the plan assets are actual assets (cash or investments) and not subject to actuarial estimates. except for the estimated return, but that is different.

plan assets use another set of actuarial assumptions, which doesnt not relate to actuarial assumptions used in PBO, e.g. average life expectancy.