Every Xbox 360 game has up to 1000 achievement points available to gain, with downloadable content offering a possible additional 250 per title. Arcade titles can award a total of 200 points over 12 achievements. How these are awarded and how much each is worth is entirely up to the developer/publisher, and as a result getting 1000 points on one game can be a hell of a lot easier than getting the full set on another (See Avatar: The Burning Earth)
I came across this site a couple of weeks back when it was in beta, and its now its open for public registration.
TrueAchievements.com works on the logical principle that the harder an achievement is to obtain, the more points it should be worth, so based on your current Gamerscore it will work out your True Achievement Gamerscore.
Full details are on their FAQ page, but the basic idea is that the true difficulty of an achievement is calculated by the frequency of people who have it, compared with the amount of people who own the game. So using the example on the site, if 100 people own a game, but only 1 person has a certain achievement, then that achievement must be very difficult to get, and should then be marked up considerably.
Upon signing up, your Gamertag data is scanned, and your True Achievement score is calculated. My latest Gamerscore, and True Achievement Gamerscore is calculated below, with a more detailed break down here.
So, what’s your True Achievement worth?