Very strange discovery: In my business I can see a lot of websites in the serps where the keyword does not belong to the content. For example: The site is optimized for "online widget" but the site is also up for "online widgets" but the keyword is nowhere to find on the site neither a link with this keyword inside is linking to this site. And some sites are listed with www.widgest.com/widgetsonline/best.html. And the keywords in my business are very strong (+20.000.000).
Than I have discovered that all the "wrong" listed domains have bought sidewide links at ONE site at the bottom. All the sites which are listed at the bottom are listed in Google for keywords they have not optimized for. There is no rational reason for those sites to be listed in the serps for those keywords and the only reason I can see is, that ALL THESE SITES have bought the same bottom sidewide link at the same site!