Dealership new scam?!?, NADA/Blue book/black book value

[cossie1600]

I was attempting to buy a car today. In my prior experiences, blue book was useless as their trade in value is always too high and no dealers would come close to it. Black book and NADA are much closer to what the cars would sell for (most banks use NADA to determine your max. loan value). Lately whenever I go into the dealership, they always seem to reference the blue book when it comes to the value. Of course when I went to the dealership today, they offered me what the blue book offered (on the extreme low side, but whatever). What I don't understand is why the sudden change? I also don't understand how the blue book is actually lower than the black book/NADA (retail value is higher though). Upon more comparison between the three, i noticed KBB has the lowest trade in value while one of the higher in retail value. I don't know if they are just being conservative or the dealers got to them and started getting them to change their numbers since a lot of consumers were using that number. What do you guys think?

2009 370z with 20K miles with sport package 6 spd (I am not selling my car, just as a reference)

Average Condition
KBB Trade In = $22,804
NADA Trade In = $23,925
Black Book Trade In=$23,100

Good Condition (Let's assume the dealer used good even though a lot of used cars are beat)
KBB Retail = $28,634
NADA Retail = $28,025
Black Book Retail= $28,200