I believe that these values are researched through dealers networks and auctions. Any Zs going through there are not that nice and most nice ones are sold in the private sector. Thus the low values. If the dealer has to put any money in a Z, it is just too costly and most non-dealer mechanics don't want to work on them.
Integras and Civics are sought out by the dealers and with the gas crunch have gone up in value.