The PV of PBO contains all of the actual gains or losses as well as past service cost.  In order to smooth the income statement, the pension expense is using expected reuturn and amortization of gains and losses above corridor and amortization of past service cost so a defined benefit liability on their balance sheet is not the PV of PBO but PV of PBO less unrecognized losses and add unregcognized gains.  This is true for IFRS and old US GAAP prior to SFAS 158.  Currently, US GAAP is using funded status as net pension asset (liability) according to SFAS 158.  

You could read the Schweser note 2 from page 110-113 for a more detailed explanation.

这几天我又看了一下教材，您的答复是对的，非常感谢。