The reason they are different is because when you visit your 'live' site, you're going to
www.nettikirppis.com/. When your site is loaded inside Facebook, it's
www.nettikirppis.com/ (the difference is the https).
You'll need to explicitly set the https (or http) in the {JFBCComments} tag if you want it to be a specific one. Each comment box is tied to a specific URL, and when you use the {JFBCComments} tag, it simply uses the 'current' URL, which may or may not have the https in it.
Hope that makes sense,
Alex