What I mean is that, for example, the interval from 1eV to 2eV has the same length as the one from 2eV to 3eV, yet they correspond to the intervals from 1/2 to 1 and from 1/3 to 1/2 (dropping units and constants), which have different lengths.

I would assume that the difference comes from the fact that the same intervals of light will correspond to intervals of different length when plotted by frequency vs when plotted by wavelength, thus making one of them appear higher, while the other is just wider