The V8 Vantage races have a varied mix of cars, so there the guys with the best cars usually win. With the Nascar races the cars should be equal, so then it has mostly to do with the level and skill of the driver. If the levels are about the same, then it has to do with the personality of the driver, is he "hot" or "cold", and the different skill sets.
I've been running Nascar races with 15 level 20 bob's against one level 35-39 bob. Usually the 35-39 bob will win, but if towards the end of the race he calms down enough and the bobs behind catch him at the right moment when they are racing each other and are thus "hot", a few of them will catch him out and beat him to the finish line.
And among the level 20 bob's, there seem to be a few who more often than not end up close to the top, so they seem to have a better mix of skills than the others. But if their "mood arrow" is pointing down, they will probably end up losing to the others.
But roughly speaking there doesn't seem to be a huge difference among equally leveled bob's, and I think it mostly has to do with their mood at the time.