"Each achievement can be worth a certain number of individual points within a game, which makes the 6.3 billion number even more impressive—that's not a total score, that's total unlocks. If one was to sum up the nebulous amount of points awarded from each Xbox 360 achievement unlocked by Xbox Live-friendly gamers, Microsoft says you'd get a gamerscore of 176,802,201,383. That's a lot of gaming.
For the numerically obsessed, Microsoft says that an average Xbox Live user has a Gamerscore of around 11,286. To put that in perspective a bit, Xbox Live user Stallion83—who was listed in the 2010 Guinness Book of World Records for "highest Gamerscore"— sits at a score of 540,563 as of this article's writing. And it's not as if he'll be slowing down anytime soon; It's his goal to hit one million points in total."