When the width of the RadSlider is higher than 2100px, the Selected Region is looks broken.
The issue is not reproduced with Q1 2015 SP1 and the following markup: <telerik:RadSlider runat="server" ID="RadSlider1" IsSelectionRangeEnabled="true" Width="2200px" MinimumValue="0" MaximumValue="300" SmallChange="1" SelectionStart="100" SelectionEnd="200"></telerik:RadSlider>