Comparing between Alexa and Google Analytics is like comparing Apples with Oranges. First of all, Alexa is not an "Analytics" it is just a few collected data showing an Alexa Rank, which means nothing and shows traffic comparison between sites which again doesn't hold much significance.
Alexa data is collected from workstations that have the Alexa toolbar installed..so if you don't have an alexa toolbar installed and visit a site, that data isn't going to alexa. So that way the data and information provided by Alexa is flawed.