backdraft7 - 4:52 pm on Feb 16, 2013 (gmt 0)
Also consider the possibility of Google's ever changing landscape (Everflux). I have found that they appear to do things for virtually no reason at all and it's usually not intentional towards any particular site, but some sites (like my own) are positioned in such a way that they suffer regularly, but also recover regularly.
I think of it like the reels on a slot machine; you have to get many to line up in order to start ranking high. One reel off and you can drop. The problem is, nobody will ever tell you what metric each reel represents and there could be thousands of them.
They like to keep us guessing. It's part of the plan to avoid gaming their system. I don't think even the MIT blackjack team has a chance at deciphering their algo.