Spectral leakage
Energy from a single frequency smeared across its neighbours in an FFT, because the analysis only ever sees a short, chopped-off slice of the signal.
An FFT assumes the block it is given repeats forever. If a tone fits a whole number of periods into the block, the assumption holds and the tone lands neatly in one frequency bin. If it doesn't – and with real music it almost never does – the block's end doesn't meet its start, and that discontinuity shows up as energy sprayed across the whole spectrum. Quiet tones near a loud one simply disappear under the spill.
The fix is to fade each block in and out with a window function: Hann, Hamming, Blackman–Harris. Every one of them trades the same two things – a wider main lobe for lower sidelobes. Fredric Harris's 1978 paper on windows for DFT harmonic analysis remains the standard reference. It's also why every spectrogram has a window-type setting that nobody wants to touch.