When you type a keyword into a search engine like Google,  there are many factors which go into deciding what results to display. Keyword frequency is very important, especially if the keyword is in the domain name or page title, but Google also looks at things such as frequency of incoming links, PageRank (essentially a measure of the pages' importance on the web), your personal search history, and the popularity of results. The entire point is to deliver the search results that most users want in regards to the query.

Searching for the series Bleach, One Piece, Naruto reveal that at least 50% of search results link to pirated content. As is the case with Puella Magi Madoka Magica, these links often appear higher in the search engine results page (SERP) ranking than the official sites. This shows that most users searching for similar queries are looking for pirated sites.

Because Google tailors its search results to each user, a user that most often clicks on search results that lead to pirated content will be served more results leading to the same. KeepAnimeAlive.com conducted its research on a search history neutral computer 

Google does do its part, however. Searching "Fullmetal Alchemist" and other FUNimation properites will show that Google has removed links from the search results page due to complaints filed by FUNimation. You can read the full complaint sent by FUNimation at ChillingEffects.org.