I've heard many people try to explain why the flag was thrown early, but no one has addressed why it was shown to the wrong car. Vettel lead the race start to finish, but he was halfway round the circuit when the Checkered came out.
Because that would have required at least one of the three people involved to be paying enough attention to the race. If they didn't know what lap it was despite the live timing systems carrying this information and the ease of lap-charting from the position the flag-waver and race starter were in, it is reasonable to think they might also have forgotten who was leading.