How can a iframe embed an https 403 forbidden site in it?

I found a website which is able access forbidden sites in it's iframe, so I inspected the webpage and the iframe and found out that there were not much of scripting or designing but a simple iframe that was showing a frobidden site into it. I find out that the site is forbidden by copying the src of the iframe and try to access it with different browsers like chrome, Firefox, Tor with no success. Can anyone explain it to me how does that website was able to access unauthorized content in it's iframe? Note: that website has a .se (Sweden) top level domain