The track itself is not the source of the reliable information. The driver on Top Gear is not consistant, which is probably the main reason why they created the Stig in the first place. Track conditions change frequently. You do not know how much the cars are prep, if at all. Some of the cars are even used. All of these contribute to a lap time with enough uncertainity that you can't tell which is faster when results are close. Of course you can tell a ferrari is faster than a Tiburon or something, but not when times are 2-3 seconds apart. Besides different tracks favor different characteristics of a car.

Same goes to BMI. I remember one episode where they just bought a used Skyline and was having problems before the test, but they went along with it.

Nevertheless, I love Top Gear and BMI.