is this a BUG about ScrollView ??

Code:

class ScrollViewDemoPage : ContentPage
{
    public ScrollViewDemoPage()
    {
        Label header = new Label
        {
            Text = "ScrollView",
            Font = Font.BoldSystemFontOfSize(50),
            HorizontalOptions = LayoutOptions.Center
        };

        ScrollView scrollView = new ScrollView
        {
            VerticalOptions = LayoutOptions.FillAndExpand,
            Content = new Label
            {
                Text = "Sometimes page content fits entirely on "+
                       "the page. That's very convenient. But " +
                       "on many occasions, the content of the page " +
                       "is much too large for the page, or only " +
                       "becomes available at runtime." +
                       "\n\n" +
                       "For cases such as these, the ScrollView " +
                       "provides a solution. Simply set its " +
                       "Content property to your content \u2014 in this " +
                       "case a Label but in the general case very " +
                       "likely a Layout derivative with multiple " +
                       "children \u2014 and the ScrollView provides " +
                       "scrolling with the distinctive look and touch " +
                       "familiar to the user." +
                       "\n\n" +
                       "The ScrollView is also capable of " +
                       "horizontal scrolling, and while that's " +
                       "usually not as common as vertical scrolling, " +
                       "sometimes it comes in handy." +
                       "\n\n" +
                       "Most often, the content of a ScrollView is " +
                       "a StackLayout. Whenever you're using a " +
                       "StackLayout with a number of items determined " +
                       "only at runtime, you should probably put it in " +
                       "a StackLayout just to be sure your stuff doesn't " +
                       "go running off the bottom of the screen.",

                Font = Font.SystemFontOfSize(NamedSize.Large)
            }
        };

        // Accomodate iPhone status bar.
        this.Padding = new Thickness(10, Device.OnPlatform(20, 0, 0), 10, 5);


        var bug = new StackLayout {
            Children = {
                new Label {
                    Font = Font.BoldSystemFontOfSize(20),
                    HorizontalOptions = LayoutOptions.Center,
                    TextColor = Color.Red,
                    Text = "After Add Another StackLayout Here"
                }
            }
        };


        // Build the page.
        this.Content = new StackLayout
        {
            Children = 
            {
        bug,
                header,
                scrollView
            }
        };
    }
}

i run this app on my real android 4.0 phone.
after i add one more StackLayout "bug" to Content StackLayout.
the scrollView 's content overlay to bug and header.
is this a BUG ? or i do something wrong ?
thanks

Bug Picture :
http://postimg.org/image/lhulb1gof/