I checked the traffic rank of two of my sites, and the one that gets approximately one fifth of the traffic of the other has a traffic rank three times as high. Alexa seems to rely on their toolbar to get those figures, and since hardly anyone uses that thing, the results are about as reliable as trowing a dice.
Thinking about it, that's probably the main reason why they are using Google results on their site now. That may give them additional click-through data, although it's impossible to predict to what degree their countings will become more useful that way.
