MVC :: Post Action Model Parameter

i wonder something about mvc post method. for example I created a view "strongly typed" from a model (MyModel).

<%@ Page Title="" Language="C#" MasterPageFile="~/Views/Shared/Site.Master" Inherits="System.Web.Mvc.ViewPage<Demo1.Models.MyModel>" %>
for Create actionsthere is two way.
first one takes MyModel as parameter
[HttpPost]
public ActionResult cart(MyModel md, int someID)
{
//some code
//.....
return View(MyModel);
}
second one does not take parameter as model.
[HttpPost]
public ActionResult cart(int someID)
{
//some code
//.....
return View(Mymodel);
}

for first one way, after form post the form values does not delete, I mean form does not cleared,after second one form is be cleared. why this diffirence?