The reason is because Compete uses data from ISPs and some other sources so it’s accumulating data from sources other than people who just have a toolbar installed.

I came to this realization when my wife, a non-techy, was visiting sites on her computer that doesn’t have the Alexa toolbar. I started researching popular sites that housewives would visit and found that they rank well on Compete but not even close to as well on Alexa.

Just my two cents.

Thanks