Angular.js ng-click events on labels are firing twice

Given the following code using angular.js

Plunkr here: http://plnkr.co/edit/i4MAzs

HTML:

<form name="myForm" ng-controller="Ctrl">
  Try clicking on the labels. <br/>
  <label>
    Value1: <input type="checkbox" ng-checked='value1' ng-click='toggleValue1()'>
  </label> <br/>
  <label ng-click='toggleValue2()'>
    Value2: <input type="checkbox" ng-checked="value2">
  </label> <br/>
  <tt>value1 = {{value1}}</tt><br/>
  <tt>value2 = {{value2}}</tt><br/>
  <tt>fire_count = {{fire_count}}</tt>
</form>

Javascript:

angular.module('App', []);

function Ctrl($scope) {
  $scope.value1 = true;
  $scope.value2 = 'YES'
  $scope.fire_count = 0;

  $scope.toggleValue1 = function(){
    $scope.value1 = !$scope.value1;
    $scope.fire_count++;
    console.log("Clicked!");
  }

  $scope.toggleValue2 = function(){
    $scope.value2 = !$scope.value2;
    $scope.fire_count++;
    console.log("Clicked!");
  }
}

The click event will fire twice when you click on the 'Value2' label. This only happens when ng-click is attached to the label. When it's attached to the input element everything works as expected.

Can someone explain what's going on?