Were they perhaps showing the frequencies manifesting the -3dB points relative to the centre frequency / max boost point? Those frequencies are fixed (as the definitions of Q I've seen all have been
Q = Fc / (Fh - Fl)
ie center frequency divided by high minus low border (-3dB) frequency). But from your description it sounds like they (at least intended to) showed a curve for the total frequency response... If you ignore the -3dB definition of the Fh and Fl, you would get the behaviour of the graph that you describe...

But the real affected frequency range (if boost <> 3dB) - compared to the "normal" level - is smaller or larger, depending on boost. At least from what I learned in school and what I've ever seen elsewhere too... Example: this page, from a site I've found to be quite good. Not sure what happens when you go to low centre frequencies and also low Q though...