This gets a little technical. To help the website send you information more quickly, pages are kept "cached" in the webserver's memory. This saves time because the server doesn't have to access multiple files every time someone wants to read a page from the site.

You noticed the downside: the server doesn't know you just added something -- in this case, a comment -- to the page. So it sends you the version that's already in its cache, from before you added your comment.

The good news is that your comment will display the next time the server refreshes its cache, within half an hour.