Alexa bases its rankings on only 2 million users who have the Alexa Toolbar (and only who cares about traffic has an Alexa Toolbar). In addition it's very very easy to cheat Alexa ranking, so websites like mine show a ranking which is much lower than the real ranking, while some spammy sites have overestimated rankings with simple black hat SEO tools. In addition, if I want to install their widget on my website to have REAL analytics, I must pay them $10 a month!