Why are CarGurus suggested market prices much lower than the other vehicle buying sites?

https://www.cargurus.com/Cars/price-trends/

The other buying sites (many are ran and designed by and to produce sales at favourable prices for dealers. Many years ago it was not hard to get your hands on the "Black book", now its closely guarded. I go to dealer only auto auctions with a friend (dealer) and it is highway robbery the mark up on these used cars and many are wrecked.