numbox未设置步长却多倍增减

<div class="mui-input-row">  
    <label>Cups1:</label>  
    <div class="mui-numbox" data-numbox-min='0' data-numbox-max='10'>  
        <button class="mui-btn mui-btn-numbox-minus" type="button">-</button>  
        <input class="mui-input-numbox" type="number" />  
        <button class="mui-btn mui-btn-numbox-plus" type="button">+</button>  
    </div>  
</div>

如图示例代码块，没有设置步长，为啥单击加减会出现三倍加减的情况，无法理解