PHP accepting json from $http.post vs $.post

I have an angular app where I setup the following json object:

var newUser = {
    username: $scope.form.username,
    password: $scope.form.password,
    userTypeId: 1,
    email: $scope.form.email
};

I then attempted to post to my php backend:

$http.post(
    ENVIRONMENT.backendUrl + '/users/add/.json?XDEBUG_SESSION_START=PHPSTORM',
    newUser
).success(function() {
    alert("success");
}).error(function() {
    alert("failure");
});

Oddly enough, none of the variables were there in the $_POST array, nor in the php://input. But when I changed the post to use jquery instead, it work:

$.post(
    ENVIRONMENT.backendUrl + '/users/add/.json?XDEBUG_SESSION_START=PHPSTORM',
    newUser
).success(function() {
    alert( "success");
}).error(function() {
    alert( "failure");
});

Any idea on why these two ways of posting cause a difference in the $_POST value?