He's swapping billets and claiming the swapped billet is what Simon originally wrote. Simon is amazed because he was just told he would find himself confused, and he is, since the fake billet is written in his handwriting (forged). Which is the simpler explanation, sleight of hand and handwriting forgery, or hypnotic memory implantation spanning several days?
Swapping billets doesn't add up. Derren talked to Simon on the phone with instructions to sign it and make sure no-one steals or switches it, and Simon opens it on camera straight from his wallet. So to switch it someone must have met Simon, and either figured out or pushed the exact same envelope/pen/signature location that Simon used, then went and had someone forge some new note the exact same, and then met Simon _again_ and somehow switched it while somehow Simon is convinced that no one has had access. This also must have happened well before the show since they already had to setup the room with BMX bike.
I mean or he could have just paid Simon, the actor, to act surprised.
