Quantcast is amazingly accurate

Most people use Alexa to figure out how popular a site is. It used to be the only option on the Internet world. But something on Alexa pushaed me to search for other sources:
1. It only tells relative ranking of a site, no estimated traffic volume.
2. Only top 100,000 sites have historical data graph.
3. Its traffic ranking data is from only alexa toolbar users, so it's easy to manipulate the result.

Two of best known substitutes of Alexa are compete.com and quancast.com. To find out which one is better, I tried three sites which I have access to their actual traffic data:

Alexa Compete(people/m) Quancast(people) Actual(visitors/m)
site 1 7217 for US 420403 378.9K 332939
site 2 22531 for US 200804 170.2K 180372
site 3 68295 for US 20311 9K 42000
site 3 is very new so the results from all three sources are not even close. However, for long time being

site 1 and 2, quancast.com works better. I am wondering how it collects and analyzes data to get such an amazing accuracy. Although it allows site owner to paste a special tag and configure audience profile so Quancast.com will get more accurate data, I doubt how many site owners will do so. It claims that the company was founded by "a team of engineers and mathematicians " though both co-founders have only CS degrees, I guess it must have a better statistical model to predict based on limited sample date.