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Geeky at the Lake of Zurich

Alexa updating its algorithm?

I sometimes check the Alexa rank for several sites and compare them to each other. My site had for the past several weeks a rank of about 59’000. Today, my rank jumped up to 49’576, that is plus 10’000 ranks. Other sites, like www.laax.com has jumped from rank 1.4 mio to 429’124 (good news for our CEO :-)), the same goes for our summer page www.flims.com. I have the webstats for those sites and they do not show any increase in users and visits in the past weeks.
And our Linux User Group website (3 weeks old) http://lugr.ch jumped from a rank above 4 millions to 778’544 and the 1wk. Avg. predicts an even higher rank (507’143) with only 610 visits in August. 

So now my question, did Alexa somehow changed/updated its algorithm or system? Or are there any other reasons for the update and climbing of the sites? Technorati and Tailrank have no information.