But as far as I can tell, that’s not really true — or, at least, it’s misleading.
Spectral Selections are drawn in typical “box” selection style — visually, the user draws a rectangle around the portion of the image they wish to select. That means that, when drawing a Spectral Selection, ultimately the point where the selection was started will either be the upper or lower corner of the rectangle (depending whether the user completes the selection by dragging upwards or downwards to define the opposite corner).
IOW, in this image (also from the manual) of the resulting selection:
The starting point for the selection was either the upper frequency boundary, or the lower frequency boundary. The center frequency has nothing to do with it, really, and the initial position of the mouse when starting the selection is unrelated to the center frequency — it’s the width (well, height) of the selection that determines where its center frequency will be.
(Splitting this into two parts, due to the new-user media restriction…)
In fact, the rest of the manual goes on to correctly document that behavior:
As it says, “the selection has been dragged from about 5,000 Hz to the bottom of the track”. IOW, the selection is defined by its upper and lower boundaries.
So, why the misleading initial mention of ‘hover at a vertical position that you want to be the approximate center frequency to act on’? That sets up an incorrect understanding of Spectral Selection in users’ minds, which the rest of the documentation and their own experiences then have to work to undo. (At least, it took me some time to realize that, despite what the manual says, drawing new Spectral Selections has absolutely nothing to do with center frequencies.)
The behavior that you comment on has been that way ever since Spectral Selection and Spectral Editing was introduced for version 2.1.0 of Audacity. (I just testing on various versions of Audacity back to 2.1.0 ).
And AFAICT the manual has always been wrong about that center frequency being the static part of the selection.
And now you’re the first person to have spotted this in the 8-9 years or so since 2.1.0 was released.
I will update the Manual for the son to be upcoming 3.4.0 release of Audacity (I cannot edit the current release Manual, sorry).