Results change depending client location

Differents results on same datacenter

[xMadx]

Hello,

As many people here, i often check my ranking on Google.
I always specify the data center to be sure, i'm located in Europe - France
ex:
[www-dc.google.com...]
[www-in.google.com...]
etc...
etc..

I'm very happy of what i'm seeing (1st or 2nd) over 7,110,000. (I tought nice job guy, the harder is done)
My next step was to understand why my position is different in Alexa, AOL and Yahoo.
And I realize that results on Google are differents if the location of the client who is making the request is located in Europe, Asia, Us or Africa.
To be sure I used differents proxy in the world, and I always get the same results if I'm connected via a proxy in Spain, Sweeden, France or Italy btu results are differents from a proxy in US or Canada (but still the same if i'm in North America).

Does anybody have notice this?
Why a such difference in ranking (I'm 1st from Europe and 25 from US)
What criterias are used by google to sort results?

Thanks you

[aliqui]

I live in South America and I have to use HotBot - AOL - Earthlink - Netscape to check positions of my sites on main Google(usa)index. I can't use Google directly anymore. Only the datacenter [www-zu.google.com...] has been showing my site's real position in google, consistent with the traffic it has being receiving

[xMadx]

I made deeper tests and i have 2 group of distinct results on the same datacenter www-in.

I used some proxy around the world and results are:

Group 1
Spain = France = Germany = Argentina = Brazil = Sweden = Switzerland = Israel = Italy = Canada = Netherlands

Group 2
Poland = United States = Great Britain = Taiwan = Centre Afrique = Australia = Japan

Why results are differents for group 1 and group 2 for the same keywords?

Cordially