Google shows different search results depending upon the viewer's IP address location. If you install the Google Global app to your browser, you can see the different results shown to a viewer from the UK, US, Canada or Australia. The rank of an individual Hub will be different when searching for the same keyword phrase query, based upon the searcher's location. It is often helpful to use Google Global to see what searchers are seeing.
If you have a Hub on the best pizza restaurants, viewers from the US expect to see results in their country instead of the UK, and vice versa. Even for subjects that might appear to surpass location, such as cancer treatment, will often show different results because medicine takes different approaches in different parts of the world.
If you specifically want more viewers from the US, use Google Global to see which competitors are ranking well for US search queries and do your research to find out why.
Another thing that factors in to Google search results is 'personalized search.' This means that Google will display results targeted to an individual's search/click-through history. There is little you can do to target people who do not turn off the personal search feature when they search on Google.