Strange behavior when splicing array, javascript

I'm working with list of checkboxes and I have next logic behavior for it:

1. if all items selected, checkbox "select all" is checked
2. if one of all selected items has been unchecked, checkbox "select all" is unchecked as well

This logic is clear. Depends of what item is checked I extract its id to an additional array and then using this array for request that to get data.

For pushing everything works fine, but for slicing the logic is strange. So I can slice the array until first item is checked, however I unchecked the first item, pushed and sliced items no more related with checkboxes.

I have reproduced plunker with it, so I appreciate if anybody could help me to find what I'm missing.

  $scope.modelClass = {
        selectedAll: false
        };

$scope.selectAllClass = function (array) {
     angular.forEach(array, function (item) {
            item.selected = $scope.modelClass.selectedAll;

            $scope.param =''

         });
    };

$scope.checkIfAllClassSelected = function (array) {
     $scope.modelClass.selectedAll = array.every(function (item) {
           return item.selected == true
        });
     $scope.checked = array.filter(function (item) {
           return item.selected == true
        }).length;

     angular.forEach(array, function (obj) {
           if(obj.selected == true){
                requestClass(obj)
            }
       });
   };

   var selectedClass = [];
   var requestClass = function (obj) {

         selectedClass.push(obj);

         angular.forEach(selectedClass, function (val) {
                 if (val.selected != true) {
                      selectedClass.splice(selectedClass.indexOf(val.id), 1);
                 }
                 else {
                      selectedClass = selectedClass.filter(function (elem, index, self) {
                             return index == self.indexOf(elem);
                     })
                 }
            });

          $scope.param = _.map(selectedClass, 'id')
     };




  $scope.classes = [
           {"id":4,"name":"Achievement","selected":false},
           {"id":13,"name":"Information","selected":false},
           {"id":6,"name":"Issue","selected":false},
           {"id":5,"name":"Message","selected":false},
           {"id":9,"name":"Request","selected":false}
           ]