Jquery 偏移量在滚动时只返回一个数字

我有一个网页，我需要获取页面元素的当前顶部位置。

我有一些div，他们每个人都有100%的高度，我在那里有我的内容。

我想从顶部获取这些div 位置并使用它。 但是当我这样做时，它只返回一个数字，并且在滚动时不会更改。

这是我的代码：

.HTML：

<header id="header">
   <div id="menu-div" class="container-full">
        <div class="container">
            <a href="#">TEST</a>
            <a href="#">TEST</a>
            <a href="#">Test</a>
            <a href="#">Test</a>
            <a href="#">Test</a>
            <a id="request-demo-a" class="pull-right" href="#">TEST demo</a>
            </div>
        </div>
   </header>

   <div id="first-div" class="center a-div">
   test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>
   </div>

   <div id="full-div" class="container a-div">
        test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>test content here...<br/>
        </div>
  <div id="other-div" class=" a-div">
  <br/>test content here...<br/>test content here...<br/>test content here...<br/><br/>test content here...<br/>test content here...<br/>test content here...<br/><br/>test content here...<br/>test content here...<br/>test content here...<br/><br/>test content here...<br/>test content here...<br/>test content here...<br/><br/>test content here...<br/>test content here...<br/>test content here...<br/><br/>test content here...<br/>test content here...<br/>test content here...<br/><br/>test content here...<br/>test content here...<br/>test content here...<br/>
  </div>

  <div id="other-div" class=" a-div">
  <br/>test content here...<br/>test content here...<br/>test content here...<br/><br/>test content here...<br/>test content here...<br/>test content here...<br/><br/>test content here...<br/>test content here...<br/>test content here...<br/><br/>test content here...<br/>test content here...<br/>test content here...<br/><br/>test content here...<br/>test content here...<br/>test content here...<br/><br/>test content here...<br/>test content here...<br/>test content here...<br/><br/>test content here...<br/>test content here...<br/>test content here...<br/>
  </div>

.CSS：

.a-div{
    height:100%;
}

#full-div{
    position:absolute;
    top:100%;
    width: 100%;
    height: auto;
    background: white;
    z-index:5;
}

#first-div{
    width:100%;
    height:100%;
    position:fixed;
    left:0;
    right:0;
    top:0;
    bottom: 0;
    margin: auto;
    color:white;
    z-index:-1;

}
#menu-div{
    background: white;
    padding:7px;
    position: fixed;
    z-index:10;
}

#menu-div .container a{
    margin-left: 15px;
    display:inline-block;
    font-size:large;
}
#request-demo-a{
    padding:5px;
    border: 2px solid #FF8000;
    color: white;
    text-shadow: none;
    background: orange;
    text-decoration: navajowhite;
}
#request-demo-a:hover, #request-demo-a:focus{
    background: #FF8000;
}

JavaScript （Jquery）：

$(function(){
        /**start:Scroll*/
        $(document).scroll(function(){
         $("#request-demo-a").html($('#full-div').offset().top);
         /// I want this return current value of full-div!
         //And when it become equel 0 do something! (This is just example)
        });
        /**End: Scroll*/      
})

JSFiddle-demo

我想获得#full-div的位置并使用它，当它等于 0 时，做某事（意味着到达顶部时），对于其他人div，我想在某个位置做某事。

但是为什么$('#full-div').offset().top滚动时只返回一个数字呢？