The magic about old PC CRTs is that they had no fixed refresh rate, and you could do weird things from 20Hz, 42Hz, ... up to 75Hz and beyond (actually, as much Hz as you wanted as long as you didn't go far off-specs regarding what the horizontal deflector could do).
And I guess, that is the reason people say "dosbox could never emulate DOS games correctly"... I would say, actually, current monitors could never display what the weird DOS games did correctly, even if you run them on a real DOS machine or in dosbox, not only regarding correct Hz, but also all the effects Demos did based on manipulating palettes in real time according to the position the electron beam was rastering, that shouldn't work on nowadays technology.
PS: Also, out of my own experience, when you start being an asshole to your CRT monitor and go off specs, you could very well drive both deflectors crazy, get a beautifully concentrated full-power electron beam pointing just to the middle of the screen, and get a pixel bright as a 100W bulb, that will illuminate your room for a couple seconds, afterwards turning black forever. True Story.