MVC Giving A Controller The Name 'AdminController' Makes It Not Work?

I am working on a ASP.NET application that uses ASP.NET MVC.I tried naming one of my controllers "AdminController" meaning I typed "Admin" in the new controller text box and it filled out the controller part all by itself of course.This controller never worked until I changed it's name. If I changed the name to anything else it worked with no problems.I looked inside my Global.asax.cs file where the routes were configured and I found no routes leading to it.I tryed adding a route to this new controller like this:

routes.MapRoute("Admin", "calcul/SomeAction",
new { controller = "Admin", action = "SomeAction" });

and it worked but then mysiteadmin would only get routed to that specific action.I renamed the controller to AdminSection and it works but I don't understand why it didn't work before.