[old_tv_nut]

Look at the Sony diagram again - 7.0 MHz is the FM frequency for mid gray, and the bandwidth of the signal is shown measured down from that. If you think about this for a while, you will see that there is more bandwidth for high frequency detail when that detail is riding on a higher brightness background, and some what less when it's riding on a dark background.

Now suppose the image has a full black-to-white fine repeating pattern in some area. In that case, the average background in that area is mid gray by definition, since it is the average of alternating black spots and white spots. This will produce a sideband frequency at the lower limit shown in the diagram. It would be impractical to draw a diagram showing every case of detail amplitude and background gray level, so systems are compared using the kind of diagram you have posted.