Accounting for Pension Plan expenses (US GAAP)

Question

Can anyone explain why, in practical terms, calculating the net pension asset or liability uses the sum of service costs & interest expenses less EXPECTED return on plan assets as opposed to ACTUAL return on plan assets?

The explanation in the curriculum suggests that this is to "smooth" returns, and that the actuarial gains are recognised in other comprehensive income.  That's fine, except it (at least in my mind) result in inaccurate reporting of the liability.

For example, what happens if the Pension plan manager vastly overestimates the investment return on the assets?  Say, expecting a return of 15% p.a. when the portfolio only delivers 5% p.a… You end up with a vastly understated Pension liability, not to mention potentially clouding management's ability to identify a mis-match between their chosen investment strategy and the Pension plan's objectives and the potential for misappropriation of pension "expenses" (i.e, increase "expected return" > increase pension fees > siphon pension fee increase to related party).

Am I missing something here?  Wondering if anyone has a clear answer (or has wondered the same thing)….