aparraga - 6:58 am on May 19, 2010 (gmt 0)

yes, I have a main keyword that ranks #4 in spain but #1 in USA, and I assume it because of the location.

There are so many parameters that G takes into account for that (the location of your server, if your domain is a .com, .us, .es, ..., the language on the page, if any address on the page (e.g. in the footer), if any currency symbol on the page, where are who link to your page from (also where are who you are linking to), the language of the anchors (or even the pages) that link to you, etc...)

The list never ends, however, the important thing here is that it's natural to have different results in the serp depending on your location.