¿Por qué la propiedad privada no tiene el mismo alcance?

I am studying module pattern with javascript and the code below is my first try, a list of tasks. I have a small issue with variable's scope.

In one of my public methods I can access the tasks variable, in another one I can't. I will try to give more details.

I have my public method "addItem" which basically does a "push" in an array tasks.push(values); in this moment it works well, my array gets the new data.

But when I try to show the same variable in my other public method "deleteTask", example: console.log('deleteTask: ', tasks);. It returns me "deleteTask: undefined"

In the second code snippet, I am showing, how I access the methods.

Well, basically I am trying to understand, why am I not able to access the private variable in "deleteTask"? And why can I in "addItem"? What the difference? It looks like simple, but I am not getting so far.

var todoModule = (function ($) {
    var tasks = [];

    function doListOfTasks() {
        for(var i=0; i<tasks.length; i++){
            $('<li>', {
                name    : 'liTask',
                id      : 'liTask'+i,
                html    : tasks[i].title + ' - '
            }).appendTo('#listOfTasks');

            $('<input>', {
                type    : 'button',
                name    : 'delTask',
                id      : 'delTask'+i,
                value   : 'Del'
            }).appendTo('#liTask'+i);

            $('#delTask'+i).data('idTask', i);
        }
    }

    function clearListTasks(){
        $('#listOfTasks').html('');
    }

    return { 
        init: function(){
            this.getListTasks();
        },

        getListTasks: doListOfTasks,

        addItem: function( values ) {
            tasks.push(values);
            clearListTasks();
            this.getListTasks();

            console.log('addItem: ', tasks);
        },

        deleteTask: function(item) {
            console.log('deleteTask: ', tasks);

            var tasks = $.grep(tasks, function(value, item) {
                        //console.log('value: ', value);
                        //console.log('item: ', item);
                        return value != item;
                    });

            clearListTasks();
            doListOfTasks();
        }
    };
}(jQuery));

Mi HTML:

Here I am accessing the public methods through events:

<script type="text/javascript">
    $( document ).ready(function() {
            todoModule.init();

            $( '#btnAdd' ).click(function() {
                todoModule.addItem({
                    title: $('#tarefa').val(),
                    date: '01/01/1999',
                    flag: 0
                });
            });

            $('ul').on('click', 'input', function() {
                var idTask = $('#'+this.id).data('idTask');
                todoModule.deleteTask(idTask);
            });
        });
</script>

I am studying this pattern through este enlace