Bootstrap的网格系统 设置col-xs-12 当屏幕小于768px时，col-xs-12没效果

[idea_however]

先上代码：

<div class="container">
    <div class="row">
        <div class="picture col-sm-6 col-xs-12 col-md-4 col-lg-4">
            <img src="public/images/banner1.jpg">
        </div>
        <div class="picture col-sm-6 col-xs-12 col-md-4 col-lg-4">
            <img src="public/images/banner2.jpg">
        </div>
        <div class="picture col-sm-6 col-xs-12 col-md-4 col-lg-4">
            <img src="public/images/banner3.jpg">
        </div>
    </div>
</div>

当屏幕小于768px时，col-xs-12无效
屏幕是1060 x 672时效果

屏幕是802 x 672 时效果

屏幕是634 x 672 时效果


请问这是什么原因