You need to map requests for your XML files to TransferRequestHandler
in web.config. Otherwise IIS will handle the request.
Jon Galloway explains how to do this here.
In summary, you add this element to location/system.webServer/handlers in your web.config:
<add name="XmlFileHandler" path="*.xml" verb="GET" type="System.Web.Handlers.TransferRequestHandler" preCondition="integratedMode,runtimeVersionv4.0" />