FSA: pension: actual asset return why, expected, return, actual

Muddahudda

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.

evolsteevol

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