fmin和fmax

[qpy7777777]

After I changed it to DFT, I found that the values of fmin and fmax would affect the magnitude of the amplitude. For example, I know that the frequency of a signal is 156.25Hz and 1562.5Hz, in which FFT=512 and sampling_rate=8000, by calculating that the index where the maximum value and the second largest value of its amplitude are located should be the frequency value. That is, 10 and 100 are calculated correctly by the original DFT, but after the linear calculation, it is found that the calculated value is inconsistent with the actual value, and the result will be affected by fmin and fmax. Why?