hmm...let's see if I can explain this in the most basic way.
attenuation at input (Vin) reduces the signal level, but does not reduce the detail of the signal (fast forier transform (FFT)will still reveal all the frequency components of the composite sine waves) such that FFT of the source output == FFT after the attenuator, only reduced in amplitude. But having a resistor divider at the output is not like atten at the input, wherein the output is driving a load resistor or load imp. The effect of a high output Z driving a low load Z is that the signal does not transfer well from the output to the load, such that the load is only sensitive to the higher signal sine wave components, whereas the lower signal sine wave components are left out. As a result your FFT at the out put of the driving stage but before the resistor divider != FFT measured after the resistor divider (assuming amp and load have formed a complete circuit)