Well, they are technically different URL's. It's possible to have
processing.org point to different locations/servers/web sites. From a usability standpoint it wouldn't make any sense to do that, but it's not impossible.
I would guess they chose the safe route and designed the bookmarks to be completely dumb in order to accommodate any not-so-intelligently designed web sites. They make no other assumptions other than "does this URL string match this URL string?" Bookmarks won't even match a sub-directory under the same domain (ex:
If there are settings tucked away in about:config to enable a more intelligent Bookmark URL checking system, I can't find them. You might try looking for a relevant add-on.