Why does Kelly Bluebook underprice cars?

Is it due to not taking into account rarity and want? I have seen many vehicles priced almost dreadfully cheap but they always sell for higher. Also this is annoying because then you get people "Quoting" the Bluebook value for your car when you KNOW it's worth well over what's stated.
Update: Also generally the only cars of mine that sell for TOP KBB are the really crappy reamed out ones. Generally they sell for well above what's quoted.
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