Note that the RGB response curves are clipped near the peaks giving a flatter response.
The result is that the intensity has two peaks in it corresponding to yellow and cyan which are brighter by nature. The colors red, green and blue are the corners of the color cube which are closer to black while yellow, cyan and magenta are corners closer to white and hence brighter. Physically the energy density at each wavelength in incandescent light is about the same for thermal radiation at 3000 °K and one would expect a similar response at all frequencies. The eye is most sensitive to green and one would expect a stronger response there.
The conclusion is that the camera does not respond in the same way to light that is underexposed as it does to a normal exposure. The colors in shadows may be shifted somewhat. One does not see the colors yellow or cyan in the underexposed spectrum. The underexposed images give a better indication on how the RGB filters work.
edit: Photopsins are the protein-pigment complexes which determine the human eye's response to light.