Simple answer. Alexa is junk. Do not even look at it. Only for the most active sites will the Alexa number mean anything. Otherwise, it will be a misleading metric that should not be paid attention to.

The reason is that Alexa has to extrapolate their results based upon a self-selective group. Alexa says that it's toolbar users are diverse, however, clearly it is a self-selected group regardless - meaning that the data sample is provided by only those who would install the Alexa toolbar. In the statistics game, that is a disaster. Only random samples work. Even then, the larger the sample size the better. Think about it. Who installs the Alexa toolbar anymore? Especially when they get a new computer? The Alexa audience is becoming narrower as fewer people install the toolbar and those who do fit within a smaller group of those who would. It was a flawed system right from the start. At least from a statistics analysis point of view.

To that end, sites with less traffic have less sample data to make any clear indication. This is why Alexa varies so widely and hence why you should not pay attention to the metric at all.