Well, they can't really say so clearly exactly what they did. In an effort to stop people from gaming it, they keep their exact algorithm hidden. When people get around it, they tweak it further, in an endless cycle.
My guess is that their solution isn't actually based on customer's true opinions of the vendor, and thus the solution is cheatable. Hiding the solution is probably their best option at this point.
The people who are trying to game google, know exactly what the solution from google is, or will find out within weeks. The ones left in the dark are the others, that is, the innocent, or the people who are not trying to cheat google, who equally, can not hold google to account.
Thus, their secrecy serves no purpose but to further their own interests.
My guess is that their solution isn't actually based on customer's true opinions of the vendor, and thus the solution is cheatable. Hiding the solution is probably their best option at this point.