Ultimately, those sites don't have a true market adjustment for limited production cars like the marauders because their values don't reflect current asking prices for these cars.
If you walked up to a seller that has a clean titled marauder with 100k on it and told him you'd give him $6k, he'd laugh at you. If you say that is what KBB says its worth, he may pee himself laughing so hard.
If you look at auto-trader, etc., at the current marauders for sale, the asking prices are no where near what KBB/NADA says they are.
And prices vary between the sites. I checked what they say mine is worth (2003 with 16k on it):
KBB: $11.3k for private sale
NADA: $8k retail.
autotrader: 2 offered at $18k with 30k-40k on the clock...that is a far cry from KBB/NADA values