I don't know if you knew this, but google don't display same results for google.com results.
Im not talking about google.us vs google.co.uk, but google.com in us and google.com in uk
If you are not resident of US and your site targets US visitors, then if you check results in your country, let say Italy, you will not have same results for "keyword" in these countries.
To see results in us or any other country you need to add this "&gl=US" for united states or "&gl=UK" for United Kingdom etc., at the end of the search string after you hit the search button.
These are some examples for keyword "bmw"
in us:
in uk