Yeah, there's no real-world physics reason for it. Like bojo said, it has to do with the accuracy of the simulation. Basically how it works is that the simulation is broken up into "steps." At each step the simulation calculates the new position of each body. When each step represents a relatively small amount of time, and the positions are updated frequently, the simulation is accurate.
When the steps are far apart, for reasons that I won't get into (because I don't understand them well enough to explain them myself
) the accuracy breaks down. As a sort of very simplistic explanation, imagine that you're drawing a circle by connecting dots. The more dots you use, the rounder it looks.