Test case:

        var Test = function () {
                this.prop1 = true //shows up in navigator pane
	};

	Test.prototype.addItems = function() {
		this.prop2 = true; //doesn't show up in navigator pane
	}

As scope can be assigned dynamically in javascript "this" won't necessarily always refer to an instance of Test. But in the Test constructor function netbeans assumes that "this" refers to a Test instance, so it makes sense to make the same assumption within prototyped methods.