Phantom, in team editor for two cars have exactly the same performance, you must have the same power for the cars and the same hability for the drivers and, the last column, must be zero, because that numbers in the last column is the variance for each driver, so, if its not zero, as an example, if a driver have a performance of 15300 and in the last column you have 280, it means that his performance will be aleatory choose by the game from a range between 15020 and 15580, so, if you have the car with the same power but the drivers have this difference, they will not perform exactly the same all the time, as the game can choose one driver with the maximum of performance range and the other with the minimum in the same race.
The variance number is exactly to try to represent how constant the driver is along the championship, as the real life, some races the driver runs better than in others.