Hi rejetto,
I think I've found the answer. It lies in the change MS made to the Default Program scheme from Vista on. Now the defaults are located in HKEY_CURRENT_USER\Software\Microsoft\Windows\Shell\Associations\UrlAssociations\http|https, and use Progid's to drive the command (mine says "Opera Protocol". So while my HKCR says Chrome, Windows never checks it. I suppose that is why Opera doesn't bother to change it, but that makes for confusion.
Here's an informative article on the problem:
http://newoldthing.wordpress.com/2007/03/23/how-does-your-browsers-know-that-its-not-the-default-browser/#comment-10553