DFT detemine frequencies
The FFT get a discrete spectrum, as a result do you have a finite number of frequency or bins, this means that FFT resolves a time waveform into its sinusoidal components, the frequency resolution is explicitly linked to the FFT size and the sample rate, the resolution frequency is:

$$ \frac{Fs}{ N}$$

Where Fs is your sample rate and N is the size of your FFT, The larger the size of the FFT more spectral component you have and it makes its results more accurate, this tells me that to get lower frequency resolution you need a large size FFT (2048, 4096).

Follow your example 8000/205 this tells me that for each FFT bin returned you may be losing precision in order 39.02439024390244Hz