Why Google.co.uk is ranking different from Google.com?

I am getting confused when comparing my keywords on gogole.com and google.co.uk
A keyword is ranked well on pos #12 google.com while google.co.uk is #35, what makes the difference?
Anyone explains for me?

Its pretty simple... The UK (co.uk) and US (.com) are different countries... Even though both are English speaking the search trends/needs are different especially when you consider geospatial related data which is the obvious big difference... Not forgetting a difference in bounce rates which has an influence for each variant of Google domain/country search ...

That said many of the results which is not geospatial related are close with rankings i.e. evergreen topics, brands, subjects given its the same term entered into search....

Its obvious when comparing countries like France, Spain etc as the language is different so the search results will differ much more and that is no different to countries with the same language although the search results will be much more closer....