Before anyone says this is a duplicate, this did not answer my question.
I was adding a drop shadow to an element, and for some reason the box-shadow was acting like the border had a different radius. But when I removed display: table-cell it rendered perfectly fine.

I also noticed that the incorrect radius of the shadow was the same radius as the outer part of the border. Here’s an image that explains it better:

#square{
    width: 20px;
    height: 20px;
    border: 2px solid #636363;
    border-radius: 4px;
    display: table-cell;
    box-shadow: inset 1px 1px 2px #5f5f5f;
}

<div id="square"></div>