Setups are mostly self-made, matching the driving style.
Someone might prefer corner entry oversteer and neutral or a bit of understeer on full throttle apex, someone the opposite.
Brake distribution differs too, someone drives carefully early race, giving 100% on final laps, needs different tires...
One might be good enough on cornering, lower downforce, another needs more
... and so on!
There are some basic principles we all know (especially those who are deeper into racing and physics) setting up suspension, gear ratios etc... next goes driving style, skills which are tottaly driver's preference.
Skagen for example, sent me his FZ2 setup for Westhill reversed... i tried it and i barely managed to do laptimes fast enough. Didnt manage to catch up my average laptime, being 2sec slower, no matter what. He's driving in completely different way than me... even if he's so fast, doesnt mean he has the super amazing setup! He has what fulfills his own needs!