Bandwidth of time-varying harmonics of speech
If the process that creates a spectrogram includes the use of a windowing function, then the artifacts of that windowing will have a nonzero bandwidth associated with that particular window function (including its length).

If a signal is modulated, then it will have a non-zero bandwidth.  Any tiny variations in the mouth cavity shape or in the amplitude of the glottal closures or in the time period between glottal closures of voiced speech could be a cause of modulation of the harmonics.  These factors seem unlikely to be a fixed constant for all humans at all times.  Which might lead one to doubt a hypothesis of fixed bandwidth.