There's a number of cars that this affects, both road cars and race cars.
The power setting you see in "settings" is the one to use as this is what the game uses to check if you are racing in power regulated races.
One car that magically gains a huge BHP boost is one of the Mazda RX8 concepts, can't remember exactly, but I think there's a 40bhp difference between what is quoted in the UCD and what is shown in settings once you've bought the car and can access that (yet doing 0 changes to the car, no oil, rebuilds, tuning parts etc - just buythe car, check power in the two different places).
I'm not sure, but I think cars like the R390 GT1 and Toyota GT1 racecars have large differences between the UCD and "settings" too, but I'm not sure, I bought these months ago and can't remember what they came with.
A bit annoying if you are doing power / weight regulated racing (instead of PP racing), as this can sometimes mean a car you think is legit, turns out not to be. Nowadays, with the power limiter it's not such a problem though, before this was introduced it was a right pain in the a55 sometimes.