When doing FFT there are several limits. and al sorts of noise shows up in the final result. In general, calculating a FFT over more samples gives a more accurate result. I've got an older Rigol DS1052E, and it is capable of showing a TFT graph, but it's resolution is too low to be useful in any way.
The (newer?) Siglent scopes have a much better FFT function then the Rigols. By calculating results over millions of samples they apparently get a dynamic range of some 80dB out of an 8 bit ADC, but to be fair, I do not even understand how that is possible. (You normally only have about 6dB for each bit (Each bit doubles the resolution)).