Why is creating a uint8 more expensive than a uint256?

Question

Assume a contract:

contract A {

    function b() constant returns (uint8 result) {
      result =1;   
    }
}

When I replace uint8 result with uint256 result then the creation of the contract gets 804 Gas cheaper:

        gasUsed
uint8     47631  
uint256   46827 

How does this work?