Yahoo Canada vs. Yahoo USA

[webmastero]

I just recently launched a web site with a very domain friendly url. The preferred keyword is in my url, so I knew that this would prove to be of some benefit when it came to the search engine. However, I am now very confused as to why it seems to be A LOT more beneficial when it comes to Yahoo Canada vs Yahoo UK - really all foreign countries vs USA. For example, my most preferred keyword ranks a 12 serp in canada, a 2 in the UK, and 416 in USA!! I had no idea that there would be this dramatic of a difference from country to country in the search engine. Has anyone else experienced this and does anyone know why this is happening?