It is about Deutsche Bank. The repayment rate can be freely changed between 1% and 5% during the fixed interest period (2x during the fixed interest period), provided that the entire loan is repaid within 40 years mathematically. That is how Deutsche Bank calculates it.
Of course, I save interest every month the lower the interest rate is.
If I pay, for example, €400 interest monthly at the beginning after full disbursement at a 2% repayment rate with an interest rate of 1.7%, then I pay less than €400 interest at an interest rate of 1.6% with the same repayment. That way, you save real money on the interest while the repayment remains the same.