一道this指向问题

var a = 1;
function F1() {
  var a = 2;
  console.log(this.a + a);  // this -> Global
}
function F2() {
  var a = 10;
  F1();
}
F2();     // 3

var Fn = function() {
  this.a = 10;  // this -> Fn
}
Fn.prototype = {
  a: 4
}
var fn = new Fn();
F1.call(fn);   // 12