There isn’t actually a 12th achievement; Osmos only has 11. Games on GFWL require a minimum of 12 achievements, but MS told us their backend would correctly handle only 11. Sadly though, for some reason Osmos on GFWL is showing a 12th, 0-point, secret “dummy” achievement. The full 200 points are spread across the existing 11 achievements. There was a patch in the pipe with MS shortly after launching on GFWL that MS claimed would fix the issue, but it got punted, and I'm afraid we don't yet have a timeline for removing this superfluous 12th achievement.
Glad you've been enjoying the game!