Why are we making a deal of this anyway? TG is not a very serious motoring show...now if they were all facts, and boring techncial stuff constantly then i'd care about this stuff... but really... its just for fun!
Also if this is the same driver on every car (and i'm not entirely convinced it is) then you have to consider the fact that he only gets maybe an hour or so in each car? Every car is different in the way it handles and how well the driver gets along with the car. I'm betting if I had an hour to practice running my own 350Z (stock) around their track, I could get it around the track faster than the Stig did back when they tested the Z... simply because I'm more familiar with my car.
Being a good race driver also has a lot to do with knowing the quirks of your car, and learning these things along with getting good "muscle memory" to be quick and accurate with the controls makes a sizable difference.
If you wanted better lap times I suppose it would be best to get an offical manufacture test driver for the specific vehicle, let them run around the track 10 or so times to get a good feel... then start recording lap times.
All i'm getting at is one person can't be perfect with everything, everyone has their specialty areas where they really accell.
And besides, you're not really trying hard enough if you don't hit the rev limiter at least once.