Again, I was studying for this topic ASC 715 as I work on pension funds and getting certified:

Funding valuation vs Accouting val. The former mostly compute cash contributions (MRC) while the latter prepare asset and liab figures to be used on financial statements including the pension footnotes.

The concept of Pension Expense is actually quite simple: you have service cost, interest cost, and you earn Expected RoR from assets and you have Amortizations. It is a typical rollforward reconciliation every year.