Jquery - Why Same Domain Ajax Request Adding Jsonp Callback Parameter

I have a simple script making a request to the server:

var DTO = { 'path': path };
var url = 'default.aspx/Get';
var test;
$('#getInstance').click(function () {
$.ajax({
url: url,
type: 'POST',
dataType: 'json',
data: JSON.stringify(DTO),
contentType: 'application/json; charset=utf-8',
success: function (msg) {
test = msg;
},
error: function (jqXHR, textStatus, errorThrown) {
alert(textStatus);
alert(errorThrown);
}
});
});

This works fine as in it connects to the server and gets the data back, with one simple problem. It is treating this request as a cross domain request, therefore using jsonp. The server code is here:

[WebMethod]
[ScriptMethod(ResponseFormat = ResponseFormat.Json)]
public static MyObject Get(string path)
{
MyObject foo = new MyObject();
return foo;
}

Normally this would not be a problem, except that I am accessing a WebMethod, and it doesnt have the capability to return a jsonp response (meaning it has no way to attach the callback function to the response. Now, if this was a manual response, I could hack it up and attach the parameter, but I am taking advantage of the built-in serialization, so no way to mess around with the response. [URL]

Let me just stress the fact that this code works. The only problem is jQuery treating this request as cross domain. But Why? UPDATE: Well, after many more hours and more testing I have narrowed this issue down to it being a bug in jquery 1.5.1. I did some testing with older versions (all 1.4 versions) and I had no problem, the request was made using JSON, and the response was received successfully. What could be the change they made that would consider this request a CORS?