Further to this, It seems that the area of change varies from disc to disc. I think that the darker scanned areas (on the top side of the disc) start where the tracks finish (on the underside of the disc).
So the less disc space used, the sooner the darker area starts on the top side of the disc. I'm not entirely sure that this is the case, but it's looking a bit that way. I think it must be something to do with the coating that is used to burn the data into the disc.

Your thoughts on this would be interesting.