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If someone in Singapore sees your site they will see Singapore ads but if the topic is US they will see US ads.
Hope this helps!
As mentioned, AdSense uses geotargeting to display ads to your users. This means ads are targeted by IP address to the location of your users.
So, if I look at your site from here in Mountain View, California, I'll see ads targeted to U.S. users. If your users in Singapore look at the same page, they'll see ads targeted to a Singaporean audience.
AdWords advertisers can choose which countries they'd like to target with their ads. Therefore, you may see U.S. advertisers targeting ads to your users in Singapore.
If you'd like to see which ads your users in other countries are seeing, you can download the Google AdSense preview tool which will allow you to do this:
Hope that helps,