When you have set the value of the RadSlider to the maximum available and have placed the control within another element, lowering the width of the parent control does not cause the RadSlider to shrink as well.

This is reproducible when the deferred dragging is enabled and the slider is defined in a template (DataTemplate or ControlTemplate). The selection values are correct, but the UI of the selection is not properly updated. 

To work this around you can subscribe for the Loaded event of slider and reset its IsSelectionRangeEnabled property.

private void RadSlider_Loaded(object sender, RoutedEventArgs e)
{
	radSlider.IsSelectionRangeEnabled ^= true;            
	radSlider.IsSelectionRangeEnabled ^= true;            
}

Wokraround: Set the Selection property in the constructor of the window using the SelectionRange<T> struct.

public MainWindow()
{
    InitializeComponent();
    this.radSlider.Selection = new SelectionRange<double>(11,16);
}