An article from Wired quoted Google's Amit Singhal, who explained that "Google Real Time search is Google's relevance technology meeting the real-time web":
Google said its real-time search offers not just a stream of data, but an organized stream filtered of spam and other irrelevant information. The key to keeping the updates relevant is that Google judges "author quality," "probability of relevance" and "query hotness," according to Singhal. "That's what real-time search is all about," he said.
From what I've seen so far, Google's real-time search shows the limitations of Google's relevance technology. Until these limitations are addressed, Google should stop cluttering the search results with irrelevant Twitter messages.