it has to do with the fact that the adress /Documents/file.html#anchor" simply does not exist. an anchor can only be interpreted by a browser program, max will open a file called "file.html#anchor" when you do this.
Oh, I thought the launchbrowser message launched the preferred web browser and then passed the URL on to the web browser. If I paste that address into the URL line in my browser, it goes straight to the location I want. I just assumed (yeah, I know) that would happen with the max launchbrowser message as well. Bummer!