Gom, that's what is hardest to understand. It is basically the Steering curve the tire travels when moving side to side. This is caused by the different pendulum effect that a single crank vs. dual crank has as it travels through the steering motion. The ackerman angles achieved on a dual crank are more effective than those achieved with a dual crank, this is why the majority of cars have gone towards this system. The exact math of it, has to be drawn and explained by someone who can elaborate on tire slip angles better than I can.
Easiest way to see the difference is with 2 chassis side by side. 1 with a uni crank and one with a dual crank.