As I was searching the forums on the net, I found that this incident has already happened previously.
Here's Alexa's explanation:
We're having a graphical display update issue, and are working on it. Our team should haveAnd I also noticed that my Alexa search bar vanished. Hmmm.
it sorted out soon. As you know, we're still collecting data. As for sites with regular Traffic Ranks vs. the Top Million list, two facts to keep in mind: 1) Traffic Rank is an average over the last 3 months and individual days are most likely going to fluctuate, and 2) the Top Million list is a daily list of the top million ranked sites that day--if your site had a great day that launched it into the top 100K that day, it'll show up there, but that day alone might not be enough to better the average sufficiently to crack into the top 100K average for the last 3 months.
We're very sorry for the inconvenience. We're still gathering all the data.
So, no worries. This was solved in the past, and will be solved. All we can do now is wait. But for how long? "We should have sorted it our soon" is pretty vague isn't it?
UPDATE: Alexa finally made an update! Here's my recent post on the update: http://digiknowzone.blogspot.com/2010/08/alexa-finally-updated-did-yours.html