I'll take a guess at this:

Since the metering function of the vacuum slide is dependent upon the butterfly opening the carb requires a more precise input from the throttle. The second (return) cable is more of a feedback control as opposed to a closing control to keep the operation consistent; this prevents any slack in the input cable and keeps the butterfly opening and closing motion smooth.

A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort. Herm Albright (1876 - 1944)