Based on the idea that the harder an achievement is to unlock (no matter its Gamerscore value) the more points it should be worth, TrueAchievements uses a formula that takes into account how many players unlocked a specific achievement. Compared to the game's total population, a difficulty multiplier is calculated. The multiplier is figured into the game's Gamerscore and -- presto! -- a new, more accurate way to calculate a person's skill ensues.
Those who earned all the Avatar achievements will be humbled to know that it's the game with the lowest TrueAchievement value.