Right, just speaking in general. I haven't played the game in awhile so I don't really remember too many details about the achievements or options, but it's good to know it won't unlock unless you're on normal. I always set it to arcade for everything and never changed it, which I guess is helpful since then when I would do a ranking run it would be much easier to deal with on normal, heh.
I think almost all of the 360 shooters have a special mode for internet ranking, or will invalidate your score if you mess with the default options.