Google detection method for sites in regional domain

How does Google detect which sites to show up in regional country version

[Imaster]

How does Google detect which sites to show up in the regional country version of Google (www.google.co.uk) apart from those sites having the ccTLD as .co.uk. For eg: One of my client's site is based in US, domain registered in US (.com), and hosted in US.

But it appears in the results for Google UK search when I select the option "pages from the UK". (Of course, they also show up when I select "the web")

The funny part is that two subdomains of the site show up in the UK results and the other two subdomains do not show up. Actually, none of them should show up when the option "pages from the UK" is selected simply because they are not UK related.

Any feedback would be appreciated.