Google is not a black box. Read Webmaster Guidelines and watch Webmaster Help Videos.
SEO is full of contradictions - things that work for other people won’t work for you, things that didn’t work for other people will propel your sites to heights undreamt of. That’s the way the search engines need it to be - because if everyone knew exactly how to position a site at #1 in the search engine results, then everyone would do it, #1 position would become worthless and so would the search engines themselves.
Google, in particular, is not as much of a “black box” as it once was. They publish clear guidelines for search, they tweet regular pieces of advice about search engine algorithm changes, and they even share the guidelines that they give their human moderators on how to grade pages.
Wait a minute, did you say human moderators?
Indeed I did. It’s not the most widely known thing in the SEO world but Google employs thousands of people worldwide to grade websites and web pages manually. These gradings are fed back into the system and are, in many ways, the raw data from which improvements to the algorithm are formed.
Google wants to give you the “best” answer and has its algorithm tuned to do just that. But was does “best” mean? That’s why there are human moderators, testing the search index against the criteria that Google have written down for what they think makes a page good (or “best”).
And, yes, you can get a copy of this document. It’s a bit of a beast, 160 pages plus in the latest version I saw, but it is well worth reading - or giving to your web developer and asking them if they’ve built your site with these in mind.
(Hint: They will probably say “yes”, so check yourself. Be your own human moderator.)