Not sure about ZL, but here in the UK the 50Hz nominal mains frequency drifts around quite a bit - perhaps a few tenths of 1%. Seen on a fairly coarse resolution spectrogram, the harmonics give a series of wavy lines spaced by 50 or 100Hz, which drift enough to give overlapping coverage during a period of a few minutes. But with very narrow resolution spectrograms, this just adds to the "wide band" noise level, rather than producing well-defined spectral lines, because the frequency does not remain within one FFT box for long enough to produce a line.