The chain rings are probably close to perfectly round when they are brand new. The thing is that the bolt holes are built with clearances so that there can be some tolerances for differently manufactured cranks. A laser precision super exact diameter bolt hole in your chain ring is no good if the person making the crank doesn't do the same thing. You would only be able to use it on specially made bikes. Also the bolts themselves are not always exactly the same diameter. So you need to have some slop in the size of your bolt holes. The result is that your chain ring can move around a bit when you tighten it down and won't be centred perfectly over the centre of your cranks.