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There are probably a bunch of other situations like this too. I think these contribute a relatively tiny number of impressions, though, so it's nothing to get overly concerned about.
When determining where to show your ads, the AdWords system looks at a user's Google interface language setting to see if it matches one of the languages that your campaign targets. For example, only users whose Google interface language is English will see ads in a campaign targeted to English.
Each Google domain's language setting defaults to the language most commonly associated with that Google domain. For example, Google.com defaults to English, Google.fr defaults to French, etc. Users can change their Google interface language via the 'Preferences' link next to the Search Bar on the Google homepage.
The Preferences link on the Google home page allows users to choose the language in which they want to search. When the user sets the language preference, Google only displays ads that are targeted to the chosen language. If the user does not specify a language preference, the Google domain (like Google.fr) determines the default language preference (in this case, French).
Since the UK and the US both speak English, there is no way to differentiate this within your account.
The Site and Category Exclusion tool lets you prevent your ads from showing on individual websites or categories of web pages in the Google content network.
We realize that all AdWords users have different preferences and that some advertisers may want to avoid appearing on web pages featuring certain kinds of content, such as UK domains. We offer this tool so that you can enjoy the benefits of advertising on the content network while still precisely controlling your targeting.
Learn how to use the Site and Category Exclusion tool at [adwords.google.com...]
If you choose the United States as your geographical targeting, that means the IP address has to be in the Unites States when they search. However, a user could be in the United States but search on Google.uk instead of Google.com. Does that make sense?
You can target by geo-graphical area (ie: United States) and by the language (ie: English) but you can't differentiate between domains (.com, .org, .edu, .uk).
I hope it is helpful.