1) From the experience of close friends, it is not an extra week. It is typically at least an extra month. Some times it's even longer.
2) It is not clear to me whether the extra time is being spent on improving the selection or simply being wasted because everyone on the committee has other things to do which introduces delay in reaching quorum+agreement.As an outsider there is no way for me to tell one way or the other, but it's something for Google to think about.
3) Humans hate uncertainty, especially in important life decisions. Many people, given a choice between an uncertain prospect of getting a job at Google and a certain job at a company that is slightly worse than Google, will choose certainty. And there are many companies who are not obviously worse than Google competing for the top talent.