This is actually easier to see on AltaVista, but its the same database as Yahoo right?

My client's site is showing up ranked very well (#1) on one of his keywords on AltaVista when the "Worldwide" option is checked. If the search is done with the "USA" option checked then his site doesn't show up at all in the first 10+ pages.

What value, setting, or meta tag, etc. is Yahoo/AltaVista using to distinquish between "worldwide" and "usa" results?

If we are doing well worldwide, why wouldn't we do well for the USA only option?

Thanks,