What is bizarre with Mr Cameron's sputter is that he arrogantly made it like a corrupt third-world country leader who orchestrates elections to remain in power perpetually. In fact, it was a shame for a British Prime Minister make such a statement, akin to what you would expect of Robert Mugabe or Saddam Hussein to say. It is utterly undemocratic for him to predict two elections before they even happen.
Come the 7th of May 2015, British voters would go to the polls; but as it now stands, we may as well not. Because Cameron has declared that he will win; probably no matter how we vote. And he would do the same in 2020 at which time he will snub our vote and move on to, perhaps, who knows what.
However, for realities' sake, any of the following may have been behind this juvenile rhetoric.