If you read the above quotation carefully, you'll see that Google relies "heavily" but not fully on computer algorithms. In a recent interview, Google's Engineering director Scott Huffman explained that a lot of human reviewers (probably 10,000) manually review Google's results:
"Every day, we are looking at a random sample of grades that we think represent the queries we get from users. Evaluators look at the quality of each result relative to those queries."
Real people take a look at Google's search results and they tell Google which websites they don't like. If your website looks spammy, it might be reported to Google by these people.
