Bootstrap更清晰的高度不同

Question

我可以使用bootstrap构建我的响应式网站，并且我使用更清晰的类来在移动设备上启用正确的清除。正如在下面的小提琴中可以看到的那样，清晰度高度在我的最后一行中有所不同 - 因为行没有完全填满。

<div class="container">
       <div class="row">
           <div class="col-xs-6 col-sm-4">
              <div class="thumbnail">
                  <img src="http://placehold.it/350x150" class="img-responsive" />
                      <div class="caption">Test</div>
               </div>               
           </div>
           <div class="clearfix visible-xs">&nbsp;</div>           
           <div class="col-xs-6 col-sm-4">
              <div class="thumbnail">
                  <img src="http://placehold.it/350x150" class="img-responsive" />
                      <div class="caption">Test</div>
               </div>               
           </div>
            <div class="clearfix visible-xs">&nbsp;</div>           
           <div class="col-xs-6 col-sm-4">
              <div class="thumbnail">
                  <img src="http://placehold.it/350x150" class="img-responsive" />
                      <div class="caption">Test</div>
               </div>               
           </div>
           <div class="clearfix">&nbsp;</div>
           <div class="col-xs-6 col-sm-4">
              <div class="thumbnail">
                  <img src="http://placehold.it/350x150" class="img-responsive" />
                      <div class="caption">Test</div>
               </div>               
           </div>
           <div class="clearfix visible-xs">&nbsp;</div>           
           <div class="col-xs-6 col-sm-4">
              <div class="thumbnail">
                  <img src="http://placehold.it/350x150" class="img-responsive" />
                      <div class="caption">Test</div>
               </div>               
           </div>
            <div class="clearfix visible-xs">&nbsp;</div>           
           <div class="col-xs-6 col-sm-4">
              <div class="thumbnail">
                  <img src="http://placehold.it/350x150" class="img-responsive" />
                      <div class="caption">Test</div>
               </div>               
           </div>
           <div class="clearfix">&nbsp;</div> 
           <div class="col-xs-6 col-sm-4">
              <div class="thumbnail">
                  <img src="http://placehold.it/350x150" class="img-responsive" />
                      <div class="caption">Test</div>
               </div>               
           </div>
           <div class="clearfix visible-xs">&nbsp;</div>           
           <div class="col-xs-6 col-sm-4">
              <div class="thumbnail">
                  <img src="http://placehold.it/350x150" class="img-responsive" />
                      <div class="caption">Test</div>
               </div>               
           </div>
           <div class="clearfix">&nbsp;</div> 
    </div>
</div>

所以问题是：

为什么最后一个clearfix的高度为190px，
而所有其他明确修正的高度为210px？