Is that true that users from region A(U.S.A) will see different ads than users from region B(Canada)?
It might happen that some advertisers choose to show their ads for a specific keyword only in the US, and then, users from Canada (and from the rest of the World) will see a different set of ad than a user from the US when they search for that keyword. AdWords advertisers can select the country where their ads will be shown. .