Deciphering ranks

The past week I've spent a lot of time researching how Alexa rates a website. This site has been online for almost three years now, and its odd that now when my readership has been increasing the Alexa rating has worsened, as has Google's Page Rank. It all seems a bit too arbitrary and quite biased to a few corporate funded sites. I subscribed to the Alexa - Web Discovery Machine blog to try and keep up with their updates.