The most common commercial search engines make their money off of advertising and then utilize an algorithm to rank the placement of results.
The results you see typically will then be in the following order:
- Advertisements
- SEO Driven Content
- Time & Popularity Driven Content
- Relevance Driven Content
So when you study and evlauate the results of a search, try to recognize the type of content as you peel back each layer of the onion.