Google Rankings Affected by Searchers Internet Address
For some time now the Google results have been biased to the location where the searcher is searching from.
This can be set by user behaviour when signed in to Google, by Location settings in Google Search Tools, or when no other information is available, by the searchers Internet Address.
Prior to 21st May 2015 setting a location under Google Search Tools took precedence over the other settings and you could control the location bias on a search by search basis
Screenshot showing location set as United Kingdom
This appears to have changed on May 21st as the ability to bias the results to the location set under Google Search Tools appears to be overridden than the location Google gathers from your Internet Address.
Screenshot showing location set to Thame (based on Internet Address) despite trying to set location to United Kingdom
What this effectively means is Google Rankings are Affected by the Searchers Internet Address and resulted are biased to the Internet Address without the end user being able to influence things.