Perfectly round objects can't really be rendered or drawn by any sort of computer program (I'm sure there are some that come close or do it, but bear with me for this), so the computer instead draws many lines equidistant from a single point. The more lines and the more blur added to the intersections of these lines, the more perfect the circle is. Unfortunately, adding more lines and anti-aliasing the final product is very taxing on a system. That's why you never see perfect circles in real-time applications like games. CGI movies on the other hand have the advantage of having things like pre-rendering of objects and environments available and use of extremely powerful machines which do better to smooth edges. In a game, a circle has to be redrawn, say, sixty times per second along with other objects, while in the movie the objects can be rendered alone and then combined once they are captured on a storage or viewing medium.
Inaccuracies are due strictly to me doing my best to reword in layman's terms.