The Pinside estimated value is probably using a good amount of older data. For a game like Addams Family, for example, the game is increasingly harder to find, which means increasing prices and less sales with recorded prices to Pinside to update the estimated value.
Ultimately there is no true "value" source or book, you need to research competed sales and take factors like condition, location, source (as-is out of a moldy basement or from an experienced shop with a warranty), etc into consideration.