From official (Alexa) website, as user chompy has send link for ( http://www.alexa.com/site/company/announcement )
all my sites ranking has gone up and i am very impressed......i know a few people who buy bunches of traffic like a 10,000 plus daily and their rank went down.....so obviously i believe that u get good Organic traffic to your site and your rank will go up....
I think not bug...But alexa has changed it's algorithm and it announced it... I found that topic here in DP... You can search in my posts as there are very low posts and you can find easily
From 20,000 to 80,000! They say it will be more accurate, but they don't explain what are their new methods to calculate datas, so I can't comment!
Too bad for webmasters! I have several websites, one has a better position others are worst. But that does not reflect the traffic. What is important is THE TRAFFIC. Alexa has just to continue its efforts!