Alexa tweaks its ranking system for more accurate results
Now Alexa is using data from "multiple sources" and not just the toolbar to create more accurate rankings. The company isn't really saying what those sources are, but many web sites will see an adjustment in their Alexa traffic reports. Alexa also isn't admitting that its rankings were incorrect before, just that they were "different."
You'll also notice that you can only get graphs for the last 9 months of traffic. Alexa is recalculating older data and will add multi-year graphs soon.