On the VIC-20 the bright color variations are less saturated than the normal colors. This may explain why on a monochrome monitor green might be actually brighter than light blue, since the peak voltages are higher. Similarly cyan may appear brighter than light purple, or even light orange. (But light orange should still be brighter than light purple.)
In your picture, the green seems too bright.
In my picture, cyan is darker than light red:
In daimansion 's picture, it looks almost ok:
There really seems to be some tolerance in how the desaturated brightnesses will look like in detail from machine to machine. As you said, maybe the different und unclean chroma fraction might be the reason.