This is a bit of a cop out answer, but Google probably sorts them based on something derived from PageRank: https://en.wikipedia.org/wiki/PageRank

In other words: according to some approximation of the authority of the page, given the graph of links to that page, and their estimated authority in the domain of the searched keywords -- it's possible (and likely, I think?) they aren't using stack overflow's vote counts in their sort at all.