Why when getting html code by view source code is different than using Java code?

I'm trying to get html content by code below:

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.URL;
import java.net.URLConnection;

public class Main {

    public static String getHtmlContent(String urlAddress) {
        URL url;
        try {
            url = new URL(urlAddress);
            URLConnection conn = url.openConnection();
            BufferedReader br = new BufferedReader(
                    new InputStreamReader(conn.getInputStream()));
            String inputLine;
            StringBuilder stringBuilder = new StringBuilder();
            while ((inputLine = br.readLine()) != null) {
                stringBuilder.append(inputLine);
            }
            br.close();
            return stringBuilder.toString();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

    public static void main(String[] args) {
        String urlEmpik = getHtmlContent("https://myduolife.com/shop/products/1/780,duolife-keratin-hair-complex.html");
        System.out.println(urlEmpik);
    }
}

The problem is that when I entered into this website and make (click by mouse) "view source code" on the website the HTML content is different than when I use my (above) Java code. This code works correctly for most websites, but there are some websites where it does not work I mean the HTML source code is different. I do not know what is the reason? Maybe it is connected with some special security on some websites? Thank you for help.