WPF - Раскадровка завершена

Я разрабатываю такую ​​игру.

class Anima
{
    Storyboard story;
    Random rand;
    Canvas canvas;
    Ellipse target;

    public Anima() {
        rand = new Random();
        canvas = new Canvas();
        target = new Ellipse();

        target.Fill = Brushes.Red;
        target.Width = 50;
        target.Height = 50;
        Canvas.SetLeft(target, rand.NextDouble() * 300);
        Canvas.SetTop(target, rand.NextDouble() * 300);
        canvas.Children.Add(target);

        story = new Storyboard();
        story.BeginTime = TimeSpan.FromMilliseconds(rand.Next(500, 5000));

        DoubleAnimation a = new DoubleAnimation();
        a.To = rand.NextDouble() * 300;
        a.Duration = new Duration(TimeSpan.FromMilliseconds(50));

        Storyboard.SetTarget(a, target);
        Storyboard.SetTargetProperty(a, new PropertyPath(Canvas.LeftProperty));
        story.Children.Add(a);

        DoubleAnimation b = new DoubleAnimation();
        b.To = rand.NextDouble() * 300;
        b.Duration = new Duration(TimeSpan.FromMilliseconds(50));

        Storyboard.SetTarget(b, target);
        Storyboard.SetTargetProperty(b, new PropertyPath(Canvas.TopProperty));
        story.Children.Add(b);

        story.Completed += new EventHandler(story_Completed);

        Window win = new Window();

        win.Loaded += delegate(object sender, RoutedEventArgs e) {
            story.Begin();
        };

        win.Content = canvas;
        win.Show();
    }

    void story_Completed(object sender, EventArgs e) {
        int next = rand.Next(500, 5000);
        double left = rand.NextDouble() * 300;
        double top = rand.NextDouble() * 300;

        Console.WriteLine("position: ({0:G6}, {1:G6})", Canvas.GetLeft(target), Canvas.GetTop(target));
        Console.WriteLine("state   : wait for " + next + " ms");
        Console.WriteLine("next    : ({0:G6}, {1:G6})", left, top);
        Console.WriteLine();

        (story.Children[0] as DoubleAnimation).To = left;
        (story.Children[1] as DoubleAnimation).To = top;

        story.BeginTime = TimeSpan.FromMilliseconds(next);
        story.Begin();
    }
}

Все отлично, но я обнаружил, что эллипс не получил правильную позицию и сделал ошибку примерно от 2% до 50%. Похоже, что событие Storyboard.Completed запускается до конца анимации.

Что с ним не так?