为什么我的下层div不在Android上居中，而在显示器上居中？

You have several divs that have fixed widths in pixels, both above and below the incorrectly centered divs. These divs are wider than the body, so the viewport automatically expands to show the full width of those elements, making it seem as though your divs are incorrectly centered. Simply replace the pixel units of the width of the too wide divs with either percents of viewport units and you'll be good to go (e.g., .grad has a width of 1280px. Change that to 100vw). For a quick and dirty fix, add this block to the top:

* {
    max-width: 100vw;
  }

编辑：其他一些答案建议您不要在响应式布局中使用绝对定位。使用position: absolute实际上是OK，只要你使用的是相对单位（例如%，em，vw），而不是固定的单位（例如px，in，pt）。