KBB is used by sellers to try to squeeze a higher price from buyers. Its unrealistic. Nobody in their right mind should be considering actually paying 6400 for a 10 year old Taurus. That example demonstrates how ridiculous KBB valuations can be.
Valuations vary considerably and the condition factors are not comparable between valuation guides. It is possible that KBB places a greater premium on lower mileages as well - which is not in the buyer's advantage because it ignores common problems with vehicles that have mostly stop & go city driving with lots of cold starts. Auction results are the best source of pricing info - but few people have access to that data.