From what I understand, it's due to a new lack of leverage since the shifter is actually shorter.
Short shifter is a bit of a misnomer. A short shifter/shift adapter strictly refers to the reduction in the travel distance of the shift (or throw). Too often people mistake the shifter height--or lack there of--and think of that as what qualifies a short shifter or they think a lowered height will reduce the throw. This IS NOT the case at all. Height is purely a preference due to many factors. What is more important is where the fulcrum is in relation to each end. This is what will determine throw distance. All a short shift adapter does is move the fulcrum point up the shifter to reduce the amount of movement (again throw) between each gear.