Mainly because I had one in my old system, I know what it's capable of, and running F.E.A.R. tip top at anything approaching 60fps isn't on its CV. Also because I know my games, and their demands, and what hardware is mustard and what's not - F.E.A.R. is a total monster of a game and requires serious hardware to run at big resolution with big frame rates, the 6800 is not up to the job. Especially at 1920x1200 - that's GX2 or SLi territory.
You may be running it at a framerate you are happy with, and that's cool. I guess it depends what you call smooth. You don't need to 'prove' anything dude, I know for a fact that a 6800 based rig cannot max F.E.A.R. even at 1024x768. Still a good card though