swiper3.0设置item个数为4个为什么刷新有时4个有时5个？

Question

所有代码都在这里，我分了4个item，但是我按F5有时4个有时5个，请问各位有碰到过这种情况吗

    var swiper = new Swiper('.swiper-container', {
        slidesPerView: 4,
        paginationClickable: true,
        spaceBetween: 30,
    });
    
    <div class="swiper-container swiper-container-teach">
        <div class="swiper-wrapper">
            <div class="swiper-slide">
                <div class="item">
                  
                </div>
            </div>
            <div class="swiper-slide">
                <div class="item">
                    
                </div>
            </div>
            <div class="swiper-slide">
                <div class="item">
                    
                </div>
            </div>
            <div class="swiper-slide">
                <div class="item">
                    
                </div>
            </div>
            <div class="swiper-slide">
                <div class="item">
                    
                </div>
            </div>
        </div>
    </div>
    
    
    .swiper-slide {
            text-align: center;
            font-size: 18px;
            background: #fff;

            /* Center slide text vertically */
            display: -webkit-box;
            display: -ms-flexbox;
            display: -webkit-flex;
            display: flex;
            -webkit-box-pack: center;
            -ms-flex-pack: center;
            -webkit-justify-content: center;
            justify-content: center;
            -webkit-box-align: center;
            -ms-flex-align: center;
            -webkit-align-items: center;
            align-items: center;
    }
    .swiper-container-teach{
        margin-top: 55px;
        .swiper-slide{
            background-color: #000000;
            width: 258px;
            height: 372px;
        }
    }