Brad,
The reason for that is because retract servos don't work in the same way as regular servos. A retract servo has 180° degrees of travel. It only goes from one side to the other. There isn't any way to "adjust" it's travel like you can with other servos. To adjust the travel of your retracts you need to do it in the manual setup of the retract itself. Either in the linkages or in the mechanism of the servo, but you won't be able to do it with your radio.

Ken