It's not so easy to explain, but it has to do with a couple of things, according to my experience.
-Steering has more resolution. With a dual bellcrank, the servo needs to travel more to achieve full throw.
-The steering angles are more efficient due to reduced scrub. This is due to the sweeping motion of the rack vs the unicrank. The dual bellcrank has less abrupt angles.
-The chassis and top deck are no longer tied together at the bellcrank. This allows the chassis to flex more and create more front grip. This in turn allows for less agressive ackerman settings which yield less tire scrub, which in turn provide higher corner speed.