According to GoogleGuide.com/Similar_Pages:
How does Google find similar pages?
By finding other sites listed on pages that link to the specified page. ... related: search operator, returns the same results as the Similar pages link.
So there is nothing that has anything to do with TLD, and everything that has to do with links that reside with the link to a particular website on the same page.
How you want to relate this to SEO is up to you I guess.
PS: I doubt it is so simplistic as GoogleGuide describe, there are probably more heuristics and criteria used by Google to find similar pages - but we will never know these criteria. (based on their blog post about discovering similar pages).