为什么我把这个div设置绝对定位，他的父容器位置会变化？

图片说明

 如题，我把第9和第12个方块设置为绝对定位，脱离文本流，可是为什么父容器container会向下挪？
不是应该两个container一样高，子容器往上移吗？现在是每绝对定位一个，父容器就往下走一格了


<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title>小练习5</title>
    <link rel="stylesheet" type="text/css" href="practice5.css">
</head>
<body>
    <div class="container">
        <div class="draggable">1</div>
        <div class="draggable">2</div>
        <div class="draggable">3</div>
        <div class="draggable">4</div>
        <div class="draggable">5</div>
        <div class="draggable">6</div>
    </div>
    <div class="container">
        <div class="draggable">7</div>
        <div class="draggable">8</div>
        <div class="draggable" style="position: absolute;">9</div>
        <div class="draggable">10</div>
        <div class="draggable">11</div>
        <div class="draggable" style="position: absolute;">12</div>
    </div>
    <script type="text/javascript" src="practice5.js"></script>
</body>
</html>

//下面是css

*{
    margin: 0;
    padding: 0;
    -webkit-box-sizing: border-box;
       -moz-box-sizing: border-box;
            box-sizing: border-box;
}
.container{
    width: 200px;
    height: 400px;
    border: 1px solid black;
    display: inline-block;
    margin: 40px;
}
.container div{
    width: 200px;
    height: 40px;
    display: inline-block;
    border: 1px solid black;
    background-color: #c55342;
}

在你的class container中添加float:left，就可以使两个container对齐了。