Classics get lower scores because they have a more differentiated audience. People are made or feel culturally obligated to read them. So the Grapes of Wrath (3.97) has a lower score than Anarch (4.62) the 15th Gaunt's Ghost novel set in the Warhammer 40,000 universe. This is because the only people who are likely to read the latter are people likely to enjoy it.

Clearly though, different movies and books server different purposes. Is Schindlers List a better movie than Airplane! Sure, but I'm not going to put on the former when I want a few laughs.