Angular $scope won't update in bootbox callback

This is my first question on SO.

When I splice an element of an array on the scope, that change is not reflected, when done in a callback of bootbox.js.

Works:

$scope.deleteA = function() {
    if (confirm("Really delete Item 3?")) {
      $scope.itemsA.splice(2, 1);
    }
}

Does not work:

$scope.deleteB = function() {
    bootbox.confirm("Really delete Item 3?", function(answer) {
      if (answer === true) {
        $scope.itemsB.splice(2, 1);
      }
    });
}

I'm mainly interested in understanding the why. This is much more important to me than having a fancy workaround.

I created a Plunker to show the effect