The problem arises when measuring the width of a frequency band, e.g. using the Measure tool on a spectrogram layer to box a vibrato cycle. The readout in semitones and cents is much smaller for higher frequencies than for lower. A real example, using a sound file of a singer: in the spectrogram layer use the measure tool to draw boxes around one vibrato cycle for each harmonic. For a fundamental at ca. 500Hz the width of the first harmonic's box is 345 cents, that of the 6th harmonic 218. (And increasing the vertical zoom increases the value displayed when you mouse over the box.) Naturally, drawing boxes around a spectrogram display can't be minutely accurate, but this seems a very large discrepancy. This is such a useful tool for musicologists discussing vibrato styles that it's a serious problem if it's not roughly accurate.