RoutedEventArgs.Source给我错误的价值？

Question

我有一个简单的WPF窗口：

<Window x:Class="WpfApplication2.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Window.CommandBindings>
        <CommandBinding Command="ApplicationCommands.New" Executed="NewCommand_Executed"/>
    </Window.CommandBindings>
    <Grid>        
        <Menu Name="muMain" Margin="0,0,0,288">
            <MenuItem Header="File">
                <MenuItem  Name="mnFileNew" Command="ApplicationCommands.New"/>
            </MenuItem>
        </Menu>
        <Button Name="btnNew" Content="Button" HorizontalAlignment="Left" Height="39" Margin="22,70,0,0" VerticalAlignment="Top" Width="439" Command="ApplicationCommands.New"/>
    </Grid>
</Window>

和后面的代码来测试ExecutedRoutedEventArgs.Source：

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    void NewCommand_Executed(object sender, ExecutedRoutedEventArgs e)
    {
        Control bt = e.Source as Control;
        Console.WriteLine("New command triggered by " + bt.Name);
    }

}

如果我点击文件 - ＆gt;新增，输出为“由 mnFileNew 触发的新命令”，然后单击按钮，输出为“由 btnNew 触发的新命令”。这一切都很好。但如果我点击文件 - ＆gt;再次换新，输出仍为“由 btnNew 触发的新命令”。谁能告诉我为什么？