The AdSense program uses geotargeting to serve Google ads to your pages - ads are specific to a user's region and language, based on their IP address. This means that the ads that are served to a user from Australia may be different than those that appear for a user from Canada. Geotargeting makes the ads even more relevant for visitors to your site, and also means that you may not see all of the ads that can be served to your pages.
If you'd like to see what ads your users from other regions are seeing, our Google Publisher Toolbar gives you the ability to preview the ads that would show on any webpage, from a number of different locations.