With competition rising in the web analytical space (Compete.com, quantcast) Alexa tries to stay relevant. It’s known that Alexa’s previous algorithm was flawed and could be gamed by downloading the toolbar on several computers and visiting your site frequently.

The Alexa Ranking system has been changed

Preliminary reports say this new method is not that much better.
Is The New Alexa More Accurate? Initial Answer: No.

The fact is Alexa is still the most referenced traffic source around and any enhancement of  the ranking method is only a good thing. At least they know they need to change to stay relevant!