Why are they applying the last-month rule to HSA changes mid way through the year, for long term existing HSA plans?

[ghaughney]

I have had a long time HSA (year 4) and am on a family plan with my child. TT is saying: We calculated for income to include under the last-month rule, but you told us a different amount. Remove your entry below to use our calculation, or leave your total if you have a situation that requires uncommon adjustments. But it asks for spouse information- which does not apply. And, last month only applies in case of changes to HDHP (not in case of a single change) from what I understand. How do I make sure I get the appropriate credit?

[BillM223]

TurboTax automatically applies the last-month rule, if the current year (now: 2025) does not have HDHP coverage for every month. Only then does TurboTax ask about the coverage in 2024 and step through the last-month rule processing.

And, last month only applies in case of changes to HDHP (not in case of a single change) - I am sorry but I don't understand what this means. What does "single change" refer to?