CSS:宽度为100%仅为50%的子元素

Question

在this page中，标题照片不能全宽显示。

​

#wrap {
  width: 990px;
}
#content-wrap {
  display: flex;
}
.image-header {
  display: block;
  width: 100%;
}
.image-header img {
  width: 100%;
  height: auto;
}
.container {
  position: relative;
  width: 960px;
  margin: 0 auto;
  padding: 0;
}

<div id="wrap">
  <div id="content-wrap" class="fluid clearfix" data-content="content">
    <!-- /#start content-wrap -->
    <div class="image-header">
      <img src="https://staging.orsgroup.com.au/eapportal/wp-content/uploads/sites/2/2016/07/ORS-Internals-960x211-EmployServOccu21.jpg" alt="Home Page" title="Home Page">
    </div>
    <div class="clear"></div>
    <div class="container">
      <div id="content" class="sixteen columns">
        <section class="post-9 page type-page status-publish hentry">

          <h1>Home Page</h1>

          <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aliquam mollis tortor vel diam volutpat luctus. Proin placerat nisl nulla, in mattis ex consectetur quis. Interdum et malesuada fames ac ante ipsum primis in faucibus. Donec vitae enim
            vel nibh hendrerit dignissim a et ante. Mauris eget tempus nunc. Donec dignissim elit sed ullamcorper semper. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Maecenas facilisis, risus eu varius lacinia, diam tortor volutpat ipsum,
            eu luctus neque massa quis leo. Phasellus blandit pellentesque justo non ornare. Cras et elit sit amet quam consequat viverra vel a diam. Pellentesque rutrum, dolor sit amet condimentum dapibus, lorem est dapibus orci, a congue ante ante ac
            lectus. Donec maximus pretium venenatis. Cras ex leo, pellentesque in libero sit amet, auctor congue sapien. Aliquam nec mi commodo ipsum sagittis tincidunt. Vestibulum sollicitudin laoreet eros vel finibus. Integer accumsan consectetur neque
            eget sodales.</p>
        </section>
        <!-- #post-## -->
        <!-- You can start editing here. -->
      </div>
      <!-- /#content-wrap -->
      <div class="clear"></div>
    </div>
  </div>
</div>

​

然而，.image-header仅显示#content-wrap宽度的50%左右。

我在#content-wrap中添加了width: 100%;，但是这个问题仍然存在。

我确实想保留flex模型。

感谢你的帮助。