Her Hypothetical Account Balance from 2023 to 2024 did not decrease, I suppose, but she can't be distributed all of it now (unless reemployed, which I doubt will happen given the circumstances that were shared with me).
Her compensation limit from 2023 to 2024 remains at $20K. The decrease in the maximum lump sum limit is coming from her being a year older (different factors being used in the calculation from last year to this year), and also the change in mortality from 2023 to 2024.
In years past, her Hypothetical Account Balance was under the maximum lump sum limit, but now her maximum lump sum limit is less than her previously accrued Hypothetical Account Balance. The normal form of payment is the lump sum equal to the account balance. Since the maximum lump sum limit has now declined, and Single Life Annuity would be calculated based on the lump sum amount, the Single Life Annuity available to her has decreased from 2023 to 2024.