It's a fictional number they needed to make ACA work with even CBO's fake accounting (Well, it's real accounting but it proceeds from a cornerstone of obviously-BS assumptions that Congress is allowed to demand they treat as unalterable facts, and then time-bounded so whatever happens after the tenth year of their cost analysis doesn't exist).
Seriously, if there was really 700 billion of fraud, waste, abuse, and untapped efficiencies in Medicare, why wasn't THAT ever addressed before, and what idiot would believe that decrementing it from the Medicare budget would make all the fraud, waste, and abuse disappear completely?