Binding issue ActualWidth on dynamic filled Grid

Question

Consider the following, simple code:

---

XAML:

<Grid Height="60" Name="grid">
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="162*" />
        <ColumnDefinition x:Name="coltest" Width="316*" />
        <ColumnDefinition Width="239*" />
    </Grid.ColumnDefinitions>
</Grid>
<Label MouseDoubleClick="TextBox_MouseDoubleClick" 
    Content="{Binding ElementName=coltest, Path=ActualWidth}" Grid.Row="1"/>

The MouseDoubleClick event:

private void TextBox_MouseDoubleClick(object sender, MouseButtonEventArgs e)
{
    grid.RowDefinitions.Add(new RowDefinition());
    for (int i = 0; i < grid.ColumnDefinitions.Count; i++)
    {
        Random r = new Random();
        Label l = new Label { Content = r.Next(10, 1000000000).ToString() };
        grid.Children.Add(l);
        Grid.SetRow(l, grid.RowDefinitions.Count - 1);
        Grid.SetColumn(l, i);
    }
}

---

My label contains the ActualWidth property of the second column via a binding. In Visual Studio I see my label containing the value 316, so the binding works.

Double clicking the label triggers its event and adds an extra row to the grid, all with a random length.

I expect to see a new value at my label, but (the at runtime calculated) 0 does not change!

What am I missing here?