Will there be a difference between caching images loaded with new Image if they are not loaded on the page and cached when they are loaded on the page?

  var img = new Image();
            img.onload = () => {
                console.log("loaded")
            }
            img.src = "https://b-ssl.duitang.com/uploads/item/201807/06/20180706150531_zFhZJ.thumb.700_0.jpeg"

Why would an image cached with the above code be re-requested when the background picture uses the same picture?

first post some of the experimental codes

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .world{
            background: url("https://b-ssl.duitang.com/uploads/item/201807/06/20180706150531_zFhZJ.thumb.700_0.jpeg");
            width: 200px;
            height: 200px;
            background-size: 100% 100%;
        }
        .box{
            display: none;
        }
        .a{
            display: none;
        }
        .btn{
            background: white;
        }
    </style>
</head>
<body>
    <div class="hello " id="div">
    </div>
    <div class="a" id="a">
    </div>
    <button id="btn">Click</button>
    <script>
        window.onload = () => {
            var btn = document.getElementById("btn");
            var div = document.getElementById("div")
            btn.addEventListener("click", () => {
                div.classList.toggle("world")
            })
            var img = new Image();
            img.onload = () => {
                console.log("loaded")
            }
            img.src = "https://b-ssl.duitang.com/uploads/item/201807/06/20180706150531_zFhZJ.thumb.700_0.jpeg"
            
        }
    </script>
</body>
</html>

here is the http package for this code

theoretically, there is only one http request after preloading the cache
, but the results of several consecutive experiments are different

the experimental method is as follows:
grab the package to open preserve log
clear the cache
refresh and then wait a period of time for click and then refresh for a while and then wait for the event point click to repeat many times

-split line-

Note: I found that when a picture is loaded in this way, the same picture will still request again from the server
my operation mode is as follows:
1, new Image preloads the picture
2, and adds a class to a div after preloading. The code is as follows:

.world{
            background: url("https://b-ssl.duitang.com/uploads/item/201807/06/20180706150531_zFhZJ.thumb.700_0.jpeg");
            width: 200px;
            height: 200px;
            background-size: 100% 100%;
        }

Why don"t background images read new Image preloaded images directly?