Tax code and Untaxed interest calculation estimate

Firstly, I received a letter giving me a new tax code today , but in the letter, it stated that I was informed of the tax I owe from earlier year but I have never received that letter . Secondly, I already received a letter in December 2024 about untaxed interest from 2023 to 2024 and I have paid the amount HMRC asked for. Now I am confused why I receive the letter today saying I have untaxed interest less adjustment for tax I owe for earlier year, which I have already paid. This is the reason now I have a new tax code. My question is, how does HMRC estimate the savings interest ? There is no break down of the calculation and I believe the calculation is over stated and the tax code is going to 'over calculate' and end up reducing my take home pay. Thank you.

Hi,
Please refer to the following link:
Tax on savings interest
Thank you.