Why are ApiController and Controller's ModelState are so different?

Recently I use ASP.net Web API to build a small project. I met some problems in returning server side model validation messages. Then I built a small test project and created a model like that:

    public class Contact : IValidatableObject
    {
        [Required]
        public string FullName { get; set; }

        [RegularExpression(@"^\d[-,\d]*\d$", ErrorMessage = "Phone number is incorrect.")]
        public string PhoneNumber { get; set; }

        [RegularExpression(@"^(\w)+(\.\w+)*@(\w)+((\.\w{2,3}){1,3})$", ErrorMessage = "Email format is incorrect.")]
        public string Email { get; set; }

        public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
        {
            if (this.Email == null && this.PhoneNumber == null)
                yield return new ValidationResult("Email and phone number can not both be empty.", new string[] { "PhoneNumber", "Email" });
        }
    }

I also created two controllers to test it. The Web API controller:

    public class ContactController : ApiController
    {
        public HttpResponseMessage Post([FromBody]Contact ct)
        {
             HttpResponseMessage msg = new HttpResponseMessage(HttpStatusCode.OK);
             msg.Content = new StringContent(Newtonsoft.Json.JsonConvert.SerializeObject(ModelState));
             return msg;
        }
    }

The MVC controller:

    public class ContactMvcController : Controller
    {
        [HttpPost]
        public ActionResult Contact(Contact ct)
        {
            return Content(Newtonsoft.Json.JsonConvert.SerializeObject(ModelState));
        }
    }

After that, I use fiddler to post a request to test it:

Http headers ----
content-type: application/json
Content-Length: 28

Http content ----
{"FullName":"Jiang Guogang"}

Then I got very different result. The MVC controller:

Noticing that all properties are contained in result whatever it has error or not. And look at the Web API controller:

That's strange. I want to know the reason why they are so different.

Personally, I think the MVC controller does better.

I also compared the prototype of the two ModelStates. They are little difference althought they are in different namespaces and libraries.

MvcController's ModelState	namespace System.Web.Mvc	System.Web.Mvc.dll
Web API Controller's ModelState	namespace System.Web.Http.ModelBinding	System.Web.Http.dll

Does that make sense?