Yes, the related operator gives some very squirrelly results these days -- in some cases. As a general rule, the more loosely related your inbound links are, the more difficulty Google has seeing important relationships. But some of the Big Daddy data looks like it may be suffering from more than off-topic backlinks.
Last year, I asked a backlink from a webmaster for whom I offered some services. She complied, generously! She gave me a site-run backlink in the footer area. The site was template-driven.
And now, although they have nothing in common, Google thinks that her site(s) are related.
I also observed that the number of pages containing a particular backlink is a critical factor in determining "related" pages. I just checked my related pages, and I saw that all related sites have more than one page with a backlink to my site, without exception. Even some directories are counted as related, as they have multiple pages (under different categories) with a link to my site.
But this is an isolated observation, and needs confirmation to be generalized.
>>I also observed that the number of pages containing a particular backlink is a critical factor in determining "related" pages.
I'm seeing this too. For example, several of the "related" sites are where my RSS feeds are located. To say my site is similar to a site that publishes RSS feeds is silly.
I see one of the my site that shows two-three UNRELATED site with the command related.
Those UNRELATED sites do not have any link pointing to my site. I am a little bit worried because those UNRELATED sites are medicine sites....
>>I am a little bit worried because those UNRELATED sites are medicine sites....
That's my question. How worried should we be if the sites are not related?
The related command shows your actuall web cluster.
If many sites link to yours, you might also see other site that the sites linking in to you are linking to.
(How to make sense of this?)
Your site: A
If B, C and D links to your site and also link to X and Y, then X and Y will be seen as related to your site. (A, B and C might or might not show up as related though, unless they have good PR.)
As mentioned it's basically a thumbnail of your linkage, it never worked as intended as it was based on the premise that linking would be topical ;-)