"new Function" is different from eval

gistfile1.js

var a = 1;

(function(num){
  var a = 10;

  // function literal
  //var f = function(b){return a + b;};

  // eval
  //eval("var f = function(b){return a + b;}");

  // Function object
  //var f = new Function("b","return a + b;");
  return f(num);
}
)(100);

// function literal 110
// eval             110
// Function Object  101 <= window.aを参照してる