Well, you said it yourself - there are two types of WD #1 buyers: the ones that want a 9.6 or a 9.8 and are willing to pay the premium, and the ones that just want a copy and aren't really concerned with the grade. That's why there's not much of a price difference amongst the lower grades.
There are a bunch of copper books that display the same characteristic ...
True but I don't understand someone paying $800 for a 7.0 CGC slab and then someone paying $1,100 for a 9.6. The price difference is odd.