I think its maybe that there are many links and every of these many connections has a bit of tolerance and play...
That and geometric inaccuracies in all chain drives (except for the basic chain drives of two identical gears and the right distance) its impossible to get a perfect fit (same in reality, its either too loose or too tight...).
That and geometric inaccuracies in all chain drives (except for the basic chain drives of two identical gears and the right distance) its impossible to get a perfect fit (same in reality, its either too loose or too tight...).