Rasmus,
Google has been doing this (changing your title tag) for a while now. Google doesn't normally just "guess" your brand name or website name, they usually get it from somewhere. I would try to find out where they are getting it from and see if you can change it there. As Andy suggests, if it's your WordPress blog, then there is a chance that they're taking it from your blog's title, and you should be able to change it.