Hmm. Well, the Stig is also reported to be the Test Driver for whatever car company Top Gear use, so he (or she?) will know the car well enough to put in a great performance to show off the car being tested.
Hmm, because he is supposed to be many different drivers! Ben Collins (google that one - it seems to have legs), Lewis Hamilton, Fernando Alonso or Damon Hill.
He used to be Perry McCarthy, but he was axed when he revealed it in his autobiography.