One of the best kept secrets at Google is that although the promotion process is described as being very fair, meritocratic and objective, in many cases promotion is given for reasons that are completely unrelated to performance.
Promotion gets an award as part of a counter offer. When you bring offers from other companies to the table on Google, they can choose to negotiate a counter-offer with you. The offer will often include a promotion to the level that you (yes, you) believe is appropriate, guaranteed to be provided during the next review cycle. I have seen this happen several times.
The committee can be biased by direct offers made by managers. If your manager really wants you to be promoted, he can appear at the promotion committee meeting directly and make his case. Obviously, not every manager does this, so candidates whose managers are at a big advantage over their peers; and given that promotions are given in bell-like curves, fashion, candidates who get promoted in this way do so at the expense of other candidates, regardless of the relative objective benefits of the candidates. You might argue that a manager's actions appear to be related to his belief in the strength of candidates, but the reality is that managers often do this for entirely selfish reasons - to prevent negative consequences threatened by candidates, such as quitting a team or company, and I have seen happened twice.