I am by no means an expert, but can try to explain it. Circular orbits are a simplification used when there is no need for high accuracy, the math is much more simpler. The problem is that celestial bodies in the real world do not follow circular orbits, they move in orbits with an elliptical shape (not always!), following Kepler's Laws. If you want to predict the position of a planet into the future, this is the model you have to use. You should check the following links, if you are interested in the topic:
I thought it might be that, except Kerbal certainly does elliptical orbits, and you'd actually have to work rather hard not to, because a basic integrator hooked up to the 1/r^2 law of gravitational attraction gives you elliptical orbits for free.
