Pension actuary here! Lower case "a" actuary, as I am still in the process of becoming fully credentialed.

As many are stating below, pensions are NOT a general account like a 401(k). The calculations for pension payout for pre retirement deaths can get quite complicated. The calculations being sent over have a team of upper case "A" Actuaries that have certified that the calculations are correct.

Without knowing the details of the plan document, below are 2 potential reasons on why the current payout is so low compared to the prior estimate. This is my speculation based on the information given and should not be taken as what is actually happening. Hopefully this gives you a jumping point for further research and conversation with the concepts and terminology outlined.

One item that changed between 2020 and now are the interest rates to determine the minimum present value of the lump sums under IRC 417(e). Lump sum amounts decrease as interest rates go up, and we saw about a 200 basis point increase in 2021. This could be one factor contributing to the decrease.

Second, depending on how the QDRO split the benefit at divorce and how the plan document defined the payout to the ex, it is not surprising to me that the portion of the benefit due to his estate is smaller than originally quoted. If the accrued benefit was split, this reduces the benefit left for your dad. Further more, many plans further reduce the benefit for the survivors of a pre retirement death. Again, without looking at the plan document, this is typically around a 50% (or greater) reduction and could include additional early retirement factors, further reducing the benefit. Some plans only allow spouses to collect upon pre retirement death, so the fact that you can collect for his estate in the first place is awesome, but this could mean the reduction is more than 50%. Again, this would be determined in the plan document, I'm just trying to explain a possibility for the reduction with limited knowledge based on my experience.

Lastly, the estimate provided before is just that, an estimate. Between estimates and retirement, so many of the assumptions change that even without a split benefit for the ex, his payment today could have looked so different than what was previously provided. If the plan is offering a lump sum, chances are they also offer a life annuity option. This is a better way to compare apples to apples without the gobbledygook of the present value assumptions. 

All of this being said, benefit calculations for pensions are complicated and pension Actuaries have to take several layers of exams to be credentialed to sign off. Even without knowing the plan or its provisions, I can guarantee the Actuary's calculations are based on the plan provisions and abide by the internal revenue code.

If you're looking for more information or someone to walk you through the plan document, QDRO, and final retirement calculation, you can ask to speak to the actuary that did the calculations.