"Because..." He tried to think, tried to make sense. Blinked a bit and fumbled to find out what the problem was. "Because it's different," he said finally.
Then he drew back suddenly, not letting go completely, but getting far enough back to see the Doctor's face, look him in the eye. "You're not him," he realised - though he'd already realised it subconsciously. He frowned. "How did you know?"