A question about pension plan surplus and actuarial return

[chris.chan5784]

Guys, a problem that occurs on the curriculum just came into mind:
that is: despite the realised return/ expected return of a pension plan is larger than the actuarial return,
the plan surplus still declines. WHY?
I met this question sometime before, but i cannot remember the answer, and i tried to search the original
question in the curriculum, but failed.
Somebody helps me?

 

[janakisri]

If liabilities rise faster than assets the plan surplus could become negative. falling rates and a structure where duration of liabilities is larger than duration of assets could lead to this

 

[chris.chan5784]

highly understandable.
thank u so much.