You miss understand the market. Yes his deal is more than the 2-3 other 7-year deals for Verlander, Sabathia, and Hernandez, but he's much younger and had better numbers.
Previous contracts are only one factor, and not the largest. The real market is what other teams would be willing to pay for Kershaw, and just about everyone expected a 10-year $300 million deal, which is what he likely would have gotten had he made it to free agency and there was competition for his signature. Especially at 26.