But the curve represents the exposure compensation applied to the image, so any variation in the curve will have an effect on the exposure. Here for example, increasing zone -4EV by +1EV will decrease zone -6EV and -2EV by -0.25EV. I think this was not intended by the devs but more a consequence of the model/math behind the curve, and also not expected by (beginner) users.
User Manual for Tone Equalizer
The exposure of each pixel of the input image is adjusted using the mask and the equalizer graph. For each pixel, the module looks up the brightness of the mask at that point, finds the matching brightness on the horizontal axis of the equalizer graph, and increases or decreases the exposure accordingly (using the vertical axis of the graph).