Because that risk is not passed on to pension beneficiaries (their benefits are guaranteed, whatever the market does) it is improper to use a liability discount rate that includes the risk premium.

FORBES

To allow the Taxpayer a refund to which it is not entitled because Tax Year 1 was previously audited would be an improper application of section 7605(b).

FORBES: NOL Gives IRS Second Chance to Disallow Deduction