Difference between Web API (standalone) and Web API ASP.NET MVC

I've created both an ASP.NET MVC project which includes the Web API and I created an empty project where only Web API was selected and I'm trying to understand why they are behaving differently.

Both of them from what I can tell have the routing definition defined as follows:

config.MapHttpAttributeRoutes();

config.Routes.MapHttpRoute(
    name: "DefaultApi",
    routeTemplate: "api/{controller}/{id}",
    defaults: new { id = RouteParameter.Optional }
);

But I'd like to understand why the functions that are defined in the controller are behaving differently.

In my ASP.NET MVC Solution, I can create multiple functions under the one controller:

public IEnumerable<Product> GetAllProducts()
{
    return products;
}

public IHttpActionResult GetProduct(int id)
{
    var product = products.FirstOrDefault((p) => p.Id == id);
    if (product == null)
    {
        return NotFound();
    }
    return Ok(product);
}

I don't have to add any attributes, well at least for gets and I can call individual function as:

http://localhost/api/test/GetProduct/1 http://localhost/api/test/GetProducts

In the Web API only solution, I had to define it as:

public IHttpActionResult HelloWorld()
{
    return Ok("Hello World");
}

but the above will not work unless I add the [HttpGet] attribute to my HelloWorld function.

Also I don't seem to be able to define multiple functions within the one controller as it is done the in ASP.NET MVC Solution. Defining the same function as above just does not work or even if I define the functions below

[HttpGet]
public IHttpActionResult HelloWorld()
{
    return Ok("Hello World");
}

[HttpGet]
public IHttpActionResult GetTime()
{
    return Ok(DateTime.Now.ToString());
}

I get the following error:

<Error>
<Message>An error has occurred.</Message>
<ExceptionMessage>
    Multiple actions were found that match the request: HelloWorld on type 
    MyProject.Controllers.TestController GetTime on type 
    MyProject.Controllers.TestController
</ExceptionMessage>
<ExceptionType>System.InvalidOperationException</ExceptionType>
<StackTrace>
    at 
    System.Web.Http.Controllers.ApiControllerActionSelector.
    ActionSelectorCacheItem.SelectAction(HttpControllerContext 
    controllerContext) at System.Web.Http.ApiController.
    ExecuteAsync(HttpControllerContext controllerContext, CancellationToken 
    cancellationToken) at System.Web.Http.Dispatcher.
    HttpControllerDispatcher.<SendAsync>d__15.MoveNext()
</StackTrace>

When I define a single function, it works but contrarily to a Web API defined in my ASP.NET MVC solution, when I call the below

http://localhost/api/test/HelloWorld

It doesn't seem to care what the function name is called as it will work if I call any of the following:

http://localhost/api/test http://localhost/api/test/GoodBye

and will only fail if I provide an invalid controller name i.e.

http://localhost/api/mytest

Can you explain the difference between the 2 and ideally what do I have to do to get a stand-alone Web API to behave the same way as it does when defined in Asp.net MVC

Thanks.

UPDATE-1

Below are 2 good examples where the behavior is clearly different but I can't see why as both are called "Web API with ASP.NET MVC".

Creating Simple Service with ASP.NET MVC Web API to be accessed by Windows Store and Windows Phone Application

Getting Started with ASP.NET Web API 2 (C#)