No two things are identical, even when every part comes from the same factory and the same team of mechanics work on both machines. In my Kart racing series we have a fleet of "identical" karts. We had a full fleet of new chassis this year and it is the most even kart racing series in Europe, if not on the planet, in terms of equal machinery. We still have the opportunity to come in on the warm up lap and dump the kart in the pitlane and grab another one. Sometimes it makes all of the difference. The slightest change can effect whether there is more oversteer or understeer, how the power is delivered, etc. We have professional test drivers and mechanics keeping the karts inline with one-another all weekend. One of the test drivers is a multi-world champion. They will always get the most out of whatever kart they drive and their equality is based on the laptimes that they can put down in them. However, not all drivers will be able to drive around a characteristic, no matter how small, that doesn't suit their style. Others can adapt. One thing is for sure, however many people change their karts, the same guys are always at the front of the championship and are always battling for the wins in every race. Where all things are equal, it is generally the driver that is the equaliser. You can go so far with the machinery, but only so far. The rest requires an adaptable human.